Furo

- MAJOR: Move theme files into a "furo" folder.
- This affects any users deriving from furo's templates.
- Add (custom) support for `pygments_dark_style`.
- Add support for `genindex` pages.
- Note that `html_split_index` is not supported at this time.
- Add support for highlighting API elements, when accessed via "hash" in URL.
- Add `language_data.js` to search page, fixing `Stemmer` related failures in Sphinx 3.4.
- Document how to add a site-wide announcement.
- Fix sphinx-copybutton placement on scrollable code blocks.
- Generate an error page, for documents using `layout.html` from Sphinx's `basic` theme.
- Translate placeholder in sidebar's search.
- Tweak how additional h1 headings are handled in ToC sidebar.
- Update dependency constraints, limiting compatibility to Sphinx 3.x versions.

- Rebuild theme assets, for main release.

- Clarify expectations around sidebar customisation.
- Declare plugin information to pacify Sphinx's "are you parallel" check.
- Disable sidebar-follows-you-as-you-scroll Javascript.
- Fix scrollbar styles affecting document body.
- Tweak colors for problematic content.
- Tweak how words wrap in sidebar title.
- Tweak spacing in API documentation.
- Tweak wrapping of text in API function/class signatures.

- Add styling for rubrics.
- Add styling for scrollbars, to make them match the theme.
- Fix bottom-of-page JS conditional.
- Fix stability of resources hashes. (thanks [dvarrazzo](https://github.com/dvarrazzo))
- Fix styling for multi-term definition lists items.
- Re-add our development documentation kitchen-sink.
- Tweak how ToC sidebar handles scrollbars.
- Tweak styling for basic definition lists.

- Fix search page. It had become non-operational, due to changes to JS
in the previous release.

- Add properly documented mechanisms for customising the sidebar.
- Add dedicated styling for ethical ads.
- Tweak how JS scripts are loaded.