Add footnotes details elements for optional clickability

[dagood]

Some footnotes can be a bit tricky to associate with the list at the end of the table, so try adding a details element to make them clickable:

This should have no content change, just formatting.

[qmuntal]

The footnotes are hard to follow, that's a real issue. On the other hand, I'm don't quite like the solution proposed here:

• Each table entry with a footnote now will have an arrow and a superscript number, which feels like too noisy.
• It is not directly evident that clicking the arrow will expand the entry, nor that the expanded content is the same as the footnote down below.
• The table layout changes when the entry is expanded, and expanding too many entries makes the table unreadable
• The markdown source code is uglier (nit)

I would prefer something like native GitHub Markdown footnotes instead, see https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/.