Publish implementers guide to Github pages

[wentelteefje]

Since the monorepo merge we are still building the Implementer's guide here, but we no longer publish it to Github pages. This can be seen by comparing the current job to the old one; notice that the line copying the book

cp -r ./artifacts/book/ /tmp/

is missing from the current job.

There's an additional complication which prevents me from just submitting a PR myself, which is the use of index-tpl-crud for creating/updating the index.html file:

index-tpl-crud upsert ./index.html ${CI_COMMIT_REF_NAME}

I'm not really sure why we use this approach, when we're only ever building the docs from master. However, I think just mirroring the old index.html is the way to go here. I'm not sure but a manual change to the current index.html to account for the new book location might already do the trick:

<h1>Polkadot Implementers Guide</h1>
	<section><ul id="impl-guide" data-gh-repo="polkadot-sdk"><li><a href="/polkadot-sdk/book">link</a></li></ul></section>

Why this is needed: At the moment the book is still hosted in the archived Polkadot repository, which means it's read-only and will no longer be maintained over there. After this issue is resolved I will setup a redirect from the current location of the book paritytech.github.io/polkadot/book to paritytech.github.io/polkadot-sdk/book.

[alvicsam]

The guide is published