rustdoc: Convert sub-variant toggle to HTML
Instead of creating a JS toggle, this injects details/summary for
sub-variants of enums. This also fixes the CSS so that the toggle button
does not jump when expanding/collapsing.
Takes inspiration from #83337 and should be considered part of #83332. Not quite sure if the `.sub-variant` selectors could be further simplified? AFAICS it is only used in that place, and that does not seem to allow any recursion.
d05e28df15