Add NBSphinx support for dark theme

[choldgraf]

This PR does a few cleanups and improvements:

• Adds support for HTML output backgrounds in the dark theme for NBSphinx outputs, similar to what we did for MyST-NB in ENH: Add dark mode support for some pydata packages #723 .
• Improves our announcement banner CSS for narrow screens
• Fixes a pa11y bug that we introduced

I don't believe that we can demo both nbsphinx and myst-nb in the same docs (they both grab the ipynb extension so can only use one). But I believe this does it and we'll need to rely on community reports if functionality breaks.

@blaylockbk want to test that this works OK for your use-case?

closes #739

[choldgraf]

Note that the broken link test is now fixed on main

[choldgraf]

I double-checked this locally and it seems to work well w/ nbsphinx:

I also added some instructions for how others could test nbsphinx locally.

I think we are good to merge this unless anybody objects!

[choldgraf]

OK I'm gonna merge this one in because it's fixing a display bug, and (at least locally) I think I've fixed it!