Add docs support to the theme

[pento]

This adds styling and functionality for the document to theme.

There are still things todo, including, but not limited to:

• [x] Styling the menu
• [x] Styling the next/previous article links
• [x] Make clicking code tabs work

[pento]

The importer currently runs manually. You can do it on your w.org sandbox with this command:

wp eval 'Import_Gutendocs::action_wporg_gutenberg_markdown_import();' --url=https://wordpress.org/gutenberg/

[pento]

The Handbook plugin on w.org automatically adds a table of contents to each page. For example, the "Topics" heading and list following it on this page:

https://wordpress.org/gutenberg/handbook/blocks/applying-styles-with-stylesheets/

I'm not really fussed whether this is styled, or removed.

[shaunandrews]

https://wordpress.org/gutenberg/handbook/blocks/applying-styles-with-stylesheets/

It looks like the code blocks are causing the content area to flow under the sidebar at smaller window widths:

--

Not really related to the styling, but the sidebar menu is missing a link back to the introduction. And, the link at the bottom of the introduction is for "Blocks," which skips the "Block API" page: