Scaladoc: Support Markdown Footnotes

[bracevac]

Scala 3's scaladoc apparently never had support for markdown footnotes so far, e.g.,
on https://scala-lang.org/api/3.3.4/docs/docs/reference/metaprogramming/macros.html
(and up to the current nightly docs) we get

But ideally, these should be rendered as proper footnotes.

[hamzaremmal]

I very much agree that this is nice to have but one should keep in mind that footnotes are technically not part of the Markdown format as of it's latest current version (0.31.2). Footnotes are a common extension to Markdown. When implemented, we should mention that scaladoc supports this extension of the Markdown format.