Add workaround for sphinx-togglebutton rtd-theme compat problem

[frankier]

See executablebooks/sphinx-togglebutton#75

[rkdarst]

Nice! That has bothered me for quite a while but I never looked enough to do something about it.

Some notes:

• Can you think of a way to condition this to only apply when the them is sphinx-rtd-theme? In theory we say that sphinx-lesson works in different themes so we shouldn't try to do anything theme-specific here.
• It could go here, which is just for sphinx-rtd-theme (style fixups we use for basically all our sites, so may as well go here): https://github.com/AaltoSciComp/sphinx_rtd_theme_ext_color_contrast/ I can copy it there myself if you think it's good.

Aside, I saw that Sphinx includes something upstream now: https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-seealso (this isn't the anchor but look at "collapseable text" right below here). I wonder if sometime it would be better to move to this instead of a separate extension.

[rkdarst]

I put it here:
AaltoSciComp/sphinx_rtd_theme_ext_color_contrast#9
Do you know that this works (do I need to test myself?)