Gallery filter styling and documentation

[brian-rose]

As discussed in #207 and #208, the fixes to get functionality back in the dropdown gallery filters (#208) still need a few things:

• better styling
• clearer documentation (for the next time something breaks)
• same fixes applied the Resource Gallery (https://github.com/ProjectPythia/projectpythia.github.io/blob/main/portal/_extensions/gallery_generator.py)