tools: add documentation regarding our api tooling

Introduces a proper imperative description of how the current API documentation build system works. Refs: nodejs/next-10#169 PR-URL: #45270 Reviewed-By: Michael Dawson <[email protected]>
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+# Node.js API Documentation Tooling
+
+The Node.js API documentation is generated by an in-house tooling that resides
+within the [tools/doc](https://github.com/nodejs/node/tree/main/tools/doc)
+directory.
+
+The build process (using `make doc`) uses this tooling to parse the markdown
+files in [doc/api](https://github.com/nodejs/node/tree/main/doc/api) and
+generate the following:
+
+1. Human-readable HTML in `out/doc/api/*.html`
+2. A JSON representation in `out/doc/api/*.json`
+
+These are published to nodejs.org for multiple versions of Node.js. As an
+example the latest version of the human-readable HTML is published to
+[nodejs.org/en/doc](https://nodejs.org/en/docs/), and the latest version
+of the json documentation is published to
+[nodejs.org/api/all.json](https://nodejs.org/api/all.json)
+
+<!-- TODO: Add docs about how the publishing process happens -->
+
+**The key things to know about the tooling include:**
+
+1. The entry-point is `tools/doc/generate.js`.
+2. The tooling supports the CLI arguments listed in the table below.
+3. The tooling processes one file at a time.
+4. The tooling uses a set of dependencies as described in the dependencies
+   section.
+5. The tooling parses the input files and does several transformations to the
+   AST (Abstract Syntax Tree).
+6. The tooling generates a JSON output that contains the metadata and content of
+   the Markdown file.
+7. The tooling generates a HTML output that contains a human-readable and ready
+   to-view version of the file.
+
+This documentation serves the purpose of explaining the existing tooling
+processes, to allow easier maintenance and evolution of the tooling. It is not
+meant to be a guide on how to write documentation for Node.js.
+
+#### Vocabulary & Good to Know's
+
+* AST means "Abstract Syntax Tree" and it is a data structure that represents
+  the structure of a certain data format. In our case, the AST is a "graph"
+  representation of the contents of the Markdown file.
+* MDN means [Mozilla Developer Network](https://developer.mozilla.org/en-US/)
+  and it is a website that contains documentation for web technologies. We use
+  it as a reference for the structure of the documentation.
+* The
+  [Stability Index](https://nodejs.org/dist/latest/docs/api/documentation.html#stability-index)
+  is used to community the Stability of a given Node.js module. The Stability
+  levels include:
+  * Stability 0: Deprecated. (This module is Deprecated)
+  * Stability 1: Experimental. (This module is Experimental)
+  * Stability 2: Stable. (This module is Stable)
+  * Stability 3: Legacy. (This module is Legacy)
+* Within Remark YAML snippets `<!-- something -->` are considered HTML nodes,
+  that's because YAML isn't valid Markdown content. (Doesn't abide by the
+  Markdown spec)
+* "New Tooling" references to the (written from-scratch) API build tooling
+  introduced in `nodejs/nodejs.dev` that might replace the current one from
+  `nodejs/node`
+
+## CLI Arguments
+
+The tooling requires a `filename` argument and supports extra arguments (some
+also required) as shown below:
+
+| Argument              | Description                                                                                                                            | Required | Example                            |
+| --------------------- | -------------------------------------------------------------------------------------------------------------------------------------- | -------- | ---------------------------------- |
+| `--node-version=`     | The version of Node.js that is being documented. It defaults to `process.version` which is supplied by Node.js itself                  | No       | v19.0.0                            |
+| `--output-directory=` | The directory where the output files will be generated.                                                                                | Yes      | `./out/api/`                       |
+| `--apilinks=`         | This file is used as an index to specify the source file for each module                                                               | No       | `./out/doc/api/apilinks.json`      |
+| `--versions-file=`    | This file is used to specify an index of all previous versions of Node.js. It is used for the Version Navigation on the API docs page. | No       | `./out/previous-doc-versions.json` |
+
+**Note:** both of the `apilinks` and `versions-file` parameters are generated by
+the Node.js build process (Makefile). And they're files containing a JSON
+object.
+
+### Basic Usage
+
+```bash
+# cd tools/doc
+npm run node-doc-generator ${filename}
+```
+
+**OR**
+
+```bash
+# nodejs/node root directory
+make doc
+```
+
+## Dependencies and how the Tooling works internally
+
+The API tooling uses an-AST-alike library called
+[unified](https://github.com/unifiedjs/unified) for processing the Input file as
+a Graph that supports easy modification and update of its nodes.
+
+In addition to `unified` we also use
+[Remark](https://github.com/remarkjs/remark) for manipulating the Markdown part,
+and [Rehype](https://github.com/rehypejs/rehype)to help convert to and from
+Markdown.
+
+### What are the steps of the internal tooling?
+
+The tooling uses `unified` pipe-alike engine to pipe each part of the process.
+(The description below is a simplified version)
+
+* Starting from reading the Frontmatter section of the Markdown file with
+  [remark-frontmatter](https://www.npmjs.com/package/remark-frontmatter).
+* Then the tooling goes to parse the Markdown by using `remark-parse` and adds
+  support to [GitHub Flavoured Markdown](https://github.github.com/gfm/).
+* The tooling proceeds by parsing some of the Markdown nodes and transforming
+  them to HTML.
+* The tooling proceeds to generate the JSON output of the file.
+* Finally it does its final node transformations and generates a stringified
+  HTML.
+* It then stores the output to a JSON file and adds extra styling to the HTML
+  and then stores the HTML file.
+
+### What each file is responsible for?
+
+The files listed below are the ones referenced and actually used during the
+build process of the API docs as we see on <https://nodejs.org/api>. The
+remaining files from the directory might be used by other steps of the Node.js
+Makefile or might even be deprecated/remnant of old processes and might need to
+be revisited/removed.
+
+* **`html.mjs`**: Responsible for transforming nodes by decorating them with
+  visual artifacts for the HTML pages;
+  * For example, transforming man or JS doc references to links correctly
+    referring to respective External documentation.
+* **`json.mjs`**: Responsible for generating the JSON output of the file;
+  * It is mostly responsible for going through the whole Markdown file and
+    generating a JSON object that represent the Metadata of a specific Module.
+  * For example, for the FS module, it will generate an object with all its
+    methods, events, classes and use several regular expressions (ReGeX) for
+    extracting the information needed.
+* **`generate.mjs`**: Main entry-point of doc generation for a specific file. It
+  does e2e processing of a documentation file;
+* **`allhtml.mjs`**: A script executed after all files are generated to create a
+  single "all" page containing all the HTML documentation;
+* **`alljson.mjs`**: A script executed after all files are generated to create a
+  single "all" page containing all the JSON entries;
+* **`markdown.mjs`**: Contains utility to replace Markdown links to work with
+  the <https://nodejs.org/api/> website.
+* **`common.mjs`**: Contains a few utility functions that are used by the other
+  files.
+* **`type-parser.mjs`**: Used to replace "type references" (e.g. "String", or
+  "Buffer") to the correct Internal/External documentation pages (i.e. MDN or
+  other Node.js documentation pages).
+
+**Note:** It is important to mention that other files not mentioned here might
+be used during the process but are not relevant to the generation of the API
+docs themselves. You will notice that a lot of the logic within the build
+process is **specific** to the current <https://nodejs.org/api/> infrastructure.
+Just as adding some JavaScript snippets, styles, transforming certain Markdown
+elements into HTML, and adding certain HTML classes or such things.
+
+**Note:** Regarding the previous **Note** it is important to mention that we're
+currently working on an API tooling that is generic and independent of the
+current Nodejs.org Infrastructure.
+[The new tooling that is functional is available at the nodejs.dev repository](https://github.com/nodejs/nodejs.dev/blob/main/scripts/syncApiDocs.js)
+and uses plain ReGeX (No AST) and [MDX](https://mdxjs.com/).
+
+## The Build Process
+
+The build process that happens on `generate.mjs` follows the steps below:
+
+* Links within the Markdown are replaced directly within the source Markdown
+  (AST) (`markdown.replaceLinks`)
+  * This happens within `markdown.mjs` and basically it adds suffixes or
+    modifies link references within the Markdown
+  * This is necessary for the `https://nodejs.org` infrastructure as all pages
+    are suffixed with `.html`
+* Text (and some YAML) Nodes are transformed/modified through
+  `html.preprocessText`
+* JSON output is generated through `json.jsonAPI`
+* The title of the page is inferred through `html.firstHeader`
+* Nodes are transformed into HTML Elements through `html.preprocessElements`
+* The HTML Table of Contents (ToC) is generated through `html.buildToc`
+
+### `html.mjs`
+
+This file is responsible for doing node AST transformations that either update
+Markdown nodes to decorate them with more data or transform them into HTML Nodes
+that attain a certain visual responsibility; For example, to generate the "Added
+at" label, or the Source Links or the Stability Index, or the History table.
+
+**Note:** Methods not listed below are either not relevant or utility methods
+for string/array/object manipulation (e.g.: are used by the other methods
+mentioned below).
+
+#### `preprocessText`
+
+**New Tooling:** Most of the features within this method are available within
+the new tooling.
+
+This method does two things:
+
+* Replaces the Source Link YAML entry `<-- source_link= -->` into a "Source
+  Link" HTML anchor element.
+* Replaces type references within the Markdown (text) (i.e.: "String", "Buffer")
+  into the correct HTML anchor element that links to the correct documentation
+  page.
+  * The original node then gets mutated from text to HTML.
+  * It also updates references to Linux "MAN" pages to Web versions of them.
+
+#### `firstHeader`
+
+**New Tooling:** All features within this method are available within the new
+Tooling.
+
+Is used to attempt to extract the first heading of the page (recursively) to
+define the "title" of the page.
+
+**Note:** As all API Markdown files start with a Heading, this could possibly be
+improved to a reduced complexity.
+
+#### `preprocessElements`
+
+**New Tooling:** All features within this method are available within the new
+tooling.
+
+This method is responsible for doing multiple transformations within the AST
+Nodes, in majority, transforming the source node in respective HTML elements
+with diverse responsibilities, such as:
+
+* Updating Markdown `code` blocks by adding Language highlighting
+  * It also adds the "CJS"/"MJS" switch to Nodes that are followed by their
+    CJS/ESM equivalents.
+* Increasing the Heading level of each Heading
+* Parses YAML blocks and transforms them into HTML elements (See more at the
+  `parseYAML` method)
+* Updates BlockQuotes that are prefixed by the "Stability" word into a Stability
+  Index HTML element.
+
+#### `parseYAML`
+
+**New Tooling:** Most of the features within this method are available within
+the new tooling.
+
+This method is responsible for parsing the `<--YAML snippets -->` and
+transforming them into HTML elements.
+
+It follows a certain kind of "schema" that basically constitues in the following
+options:
+
+| YAML Key      | Description                                                                                                                     | Example                                               | Example Result              | Available on new tooling |
+| ------------- | ------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------- | --------------------------- | ------------------------ |
+| `added`       | It's used to reference when a certain "module", "class" or "method" was added on Node.js                                        | `added: v0.1.90`                                      | `Added in: v0.1.90`         | Yes                      |
+| `deprecated`  | It's used to reference when a certain "module", "class" or "method" was deprecated on Node.js                                   | `deprecated: v0.1.90`                                 | `Deprecated since: v0.1.90` | Yes                      |
+| `removed`     | It's used to reference when a certain "module", "class" or "method" was removed on Node.js                                      | `removed: v0.1.90`                                    | `Removed in: v0.1.90`       | No                       |
+| `changes`     | It's used to describe all the changes (historical ones) that happened within a certain "module", "class" or "method" in Node.js | `[{ version: v0.1.90, pr-url: '', description: '' }]` | --                          | Yes                      |
+| `napiVersion` | It's used to describe in which version of the N-API this "module", "class" or "method" is available within Node.js              | `napiVersion: 1`                                      | `N-API version: 1`          | Yes                      |
+
+**Note:** The `changes` field gets prepended with the `added`, `deprecated` and
+`removed` fields if they exist. The table only gets generated if a `changes`
+field exists. In the new tooling only "added" is prepended for now.
+
+#### `buildToc`
+
+**New Tooling:** This feature is natively available within the new tooling
+through MDX.
+
+This method generates the Table of Contents based on all the Headings of the
+Markdown file.
+
+#### `altDocs`
+
+**New Tooling:** All features within this method are available within the new
+tooling.
+
+This method generates a version picker for the current page to be shown in older
+versions of the API docs.
+
+### `json.mjs`
+
+This file is responsible for generating a JSON object that (supposedly) is used
+for IDE-Intellisense or for indexing of all the "methods", "classes", "modules",
+"events", "constants" and "globals" available within a certain Markdown file.
+
+It attempts a best effort extraction of the data by using several regular
+expression patterns (ReGeX).
+
+**Note:** JSON output generation is currently not supported by the new tooling,
+but it is in the pipeline for development.
+
+#### `jsonAPI`
+
+This method traverses all the AST Nodes by iterating through each one of them
+and infers the kind of information each node contains through ReGeX. Then it
+mutate the data and appends it to the final JSON object.
+
+For a more in-depth information we recommend to refer to the `json.mjs` file as
+it contains a lot of comments.