doc: responsive docs, rewrite font sizes

This makes the docs much more mobile-friendly by adding a viewport meta
tag which makes mobile browers properly scale fonts. Additionally the
font sizes have been cleaned up to use `rem` units where possible. Also
included are some fixes for the version dropdown.

PR-URL: #15660
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>