Fix: #523

Collapsible cells currently look like this:

sphinx_book_theme	furo	pydata_sphinx_theme	alabaster	sphinx_rtd_theme	sphinx_immaterial

biggest problem here is that they don't handle dark mode. secondary is that they don't adapt to the theme. this means that myst-nb is not really usable out of the box for all the modern themes that have a dark mode! I've been copy/pasting the same dark mode overrides for myst-nb sphinx projects for years and i figure i might as well try to help out with the problem.

challenge: there isn't a standard way to declare colors as css vars or things we could otherwise anticipate and use when rendering a notebook - we can't reliably ship opinionated css for rendering collapsibles (and it's arguably not our job, but the job of the theme).

proposal: use admonition classes! Very simple change to the html rendering process to add classes and switch a span to a p, and then clearing out the css rules that set colors, add some rules that unset values commonly used with details and admonition tags so they take on the appearance of the theme.

	sphinx_book_theme	furo	pydata_sphinx_theme	alabaster	sphinx_rtd_theme	sphinx_immaterial
closed
open

so not 100% perfect, but imo an improvement. only breaking change to existing use will be to swap out span -> p in css overrides, but this should also give us a more stable selector to target in the future rather than needing to override by being relative to a .cell element (we could add a more unique class just for the sake of overriding, but didn't want to do too much)

edit: a more foolproof version of this would be to not use details but directly mimic the structure of admonitions and use javascript to open/close the container. i could also do that if maintainers prefer, i just figured start with this since it was easier and more semantic html5 compliant, but it does miss the styles a bit in some of these themes.