Rework the kitchen sink

[pradyunsg]

This brings in the current Kitchen sink from Furo and adapts it to being plain reStructuredText.

This is significantly more extensive than the older kitchen sink, covering significantly more content.

[pradyunsg]

I accidentally ended up pushing a couple of follow up commits to fix mistakes directly to main. 😓

• https://github.com/sphinx-themes/sphinx-themes.org/commit/97786363a5ada71e9cb77e0533b184bcb9f9a8db
• https://github.com/sphinx-themes/sphinx-themes.org/commit/fd1ba4137e9c71030185c56b2a747e7310749985

Noting here for future readers, in case someone comes around wondering what happened here.

[shirou]

Thank you and sorry, I didn't noticed that.