docs: Switch to Furo.

[FayeDel]

About

This pull request switches the theme to Furo.

Besides Furo-specific benefits (like light/dark mode toggle), this also:

• Optimises how the object is shown on initialisation.
• Includes a copy-button to directly copy example codeblocks to the clipboard instead of manually cmd/ctrl+C and V.
• Optimises enums to show all values including string Enums (like Locale).
• Includes permalinking for in-library models.

(Also, updates localisation to catch up with current release.)

Checklist

• I've ran pre-commit to format and lint the change(s) made.
• I've checked to make sure the change(s) work on 3.8.6 and higher.
• This fixes/solves an Issue.
  • (If existent): [REQUEST] Move to Furo, a better Sphinx theme. #615
• I've made this pull request for/as: (check all that apply)
  • Documentation
  • Breaking change
  • New feature/enhancement
  • Bugfix

[AstreaTSS]

https://interactionspy--938.org.readthedocs.build/en/938/

[EepyElvyra]

Merge when you're ready ;D