Improve doc build time and load time performance

[levithomason]

Build times

Webpack build and load time performance was improved some time ago reducing build times from ~16s down to ~1s. Something recently has increased the doc build up to ~27s. This is a massive decrease in performance, 27x worse than it once was.

Load times

I've also noticed the doc site load times are becoming unacceptably long.

We should investigate the bundle sizes and, at a minimum, skip sourcemaps on the live docs.

 27M    ./docs/build
660K    ./docs/build/assets
5.8M    ./docs/build/app.b7becd54b1798794f19a.js
6.1M    ./docs/build/app.b7becd54b1798794f19a.js.map
6.6M    ./docs/build/vendor.b7becd54b1798794f19a.js
7.7M    ./docs/build/vendor.b7becd54b1798794f19a.js.map

We should also consider splitting the bundle by page on the doc site to improve performance.

Development & HMR

The current hot reload transforms do not work with many of our components due to the way the dependencies are resolved with Webpack's require.context(). We should remove the extra transforms and just use vanilla HMR. This will reduce the development footprint and also restore browser reloads on change.