Furo

- Fix a bug in how stylesheets are handled.
- Clarify how to install directly from GitHub.
- Document how to install from git.

- Center figures and legends with `margin: auto`.
- Improve compatibility with `json` builder, by not passing functions into the
Jinja templates.
- Add a friendly comment to `domainindex.html`, about it not being implemented.
- Add styling for GUI labels.

- Fix wrong height on wide screens, for pages with a tall sidebar but not-tall
content.
- Add type annotations to the codebase! 🎉
- Fix an instance of missing brackets in documentation.

- Document how to inject custom code in Sphinx documentation
- Document that `pygments_dark_style` is Furo-specific
- Make `sphinx-panels` respond to dark mode with Furo
- Stop `defer`ing Javascript, which was causing search to break in some cases.

- Disable environment caching if `pygments_dark_style` is changed.
- Revert to earlier background color for inline literals, and allow configuring
it via a CSS variables.

- Fix code-block overflow issue, introduced by the fix for sphinx-copybutton compatibility.
- Tweak horizontal rules, to always be 1px tall.
- Tweak background color for inline literals, to match code blocks.