Identify, clean up and upstream CSS

[maximlt]

There's a lot of CSS lying around, both in this repo and in the various repos that use NBSite.

In NBSite:

• nbsite/_shared_static/nbsite.css: dates back to 5 years ago so there's probably lots of it that is obsolete as depending on former themes, but it also has some new rules that are worth having a close look at
• mystnb.css: in shared_conf.py there's a little function that removes the CSS file distributed by MyST-NB; instead we ship our own version with some changes (what changes?). MyST-NB has made its CSS file more configurable with CSS variables (👌 IMPROVE: Move CSS colors to variables executablebooks/MyST-NB#448), so that might no longer be required. Or it might still be required. Need more investigation!

In a project like Panel you would typically find:

• custom.css: sets the links colors (shouldn't this be done in conf.py?) and a bunch of mixed CSS rules. Since they're usually quite useful they're (partially) copy/pasted across projects.
• some more specific CSS, like dataframe.css: depending on how general are those files and the rules they contain, they may or may not need to be upstreamed