MAINT: Update CSS for external packages and remove redundant code

[choldgraf]

This PR does a few follow-ups to #471 , it:

• Updates some CSS that regressed in [WIP] REFACTOR: Re-work HTML structure and CSS #471
• Updates some of our regression tests so that they're more reliable and don't depend on multiple Sphinx versions (supercedes MAINT: Update tests to keep single Sphinx version #466)
• Updates to the latest pydata sphinx theme
• Re-factors our sidebar navigation logic a bit to use less custom python, and to not require a second parse with beautifulsoup

Todo

• Everything in Fix regressions with a few extensions #476
  closes Fix regressions with a few extensions #476
• Figure out why the pydata theme 0.8 breaks the column width for bootstrap, see content-blocks for ex.
• Fix safari bug:
  closes Fullscreen mode doesn't work in Safari #477
• See if our accessibility scores improved with the downstream updates.
  closes Get our lighthouse scores to "good" in relevant categories #468

[choldgraf]

OK I think this one is ready to go - fixed all of the little bugs we found and made some other general improvements to the structure. Will leave it open for a day or two in case others have feedback

cc @mathbunnyru - I think this fixes the safari fullscreen bug. Wanna try it at: https://sphinx-book-theme--478.org.readthedocs.build/en/478/ ?

[mathbunnyru]

@choldgraf it half-works :)
So, it goes to the fullscreen, when I press the button in Safari.
But when I press it again, it doesn't go to normal mode and stays full screen (it works well in Chrome).

[choldgraf]

ok @mathbunnyru try again!

[mathbunnyru]

Seems to work now, thanks!

[choldgraf]

OK merging this one in as the next step towards our CSS refactor!