Footnote rendering is not very nice

I did try a render of this locally and the footnote rendering we have on docs pages leaves quite a bit to be desired. Might want to follow this PR up with a style update to make it look nicer.

_Originally posted by @james7132 in https://github.com/bevyengine/bevy-website/issues/2204#issuecomment-3180769127_

Start by rendering a documentation page containing footnotes locally and inspect the current output. Identify where the docs page styles are defined, then make the footnotes visually clearer and verify the result across relevant docs pages.

Written by the indexing model from the issue text.