css: for sphinx-togglebutton, remove buffer space in collapsed admonitions

[rkdarst]

• With sphinx_rtd_theme, when an admonition (sphinx-lesson terminology: directive) is toggled closed, there is a small amount of padding at the bottom, where content would be.

• This CSS change removes that padding.

• This is designed to be relatively safe for other themes... but who knows, it needs checking. This will be done before merging.

[rkdarst]

It can be browsed here: https://coderefinery.github.io/sphinx-lesson/branch/rkdarst--toggle-style-remove-margin/sample-episode-rst/

I just checked with two other themes: sphinx_book_theme and alabaster. Both are constructed so that it this CSS doesn't affect them. In the futere, it would be better to not make this unconditional, but sphinx-lesson would look at what theme is in use and then inject the custom CSS. Or better yet, do that in the upstream sphinx-togglebutton. But that can be improved later, I think.

So, I think this is good to go.