Furo

- Add spacing around light theme / dark theme / auto theme toggle.

- Site visitors can now force light theme / dark theme, independently of browser settings. 🎉
- Rework handling of dark theme code block highlighting.
- "Hide Search Matches" shows up in the sidebar, when the user has search matches highlighted.
- Search term highlights are only shown in the page content.
- Fix styling for default aligned tables.
- Enable smooth scrolling.
- meta: Upgrade NodeJS packages and start using Babel.
- meta: Rework organisation of various user-facing CSS variables.

- Fix image alignment in Sphinx 4.

- Require Sphinx 4.
- Rework CSS/JS asset inclusion, to work better with Sphinx 4.
- Document how to add a custom admonition style.

- Fix dark mode highlighting in Sphinx 4.

- Allow use with Sphinx 4.
- Fix right alignment of viewcode links, when used with certain API
signatures.