feat: add markdown footnotes support

[thomapapavramidis]

Purpose of this pull request:
Description
Added MD/MDX footnote support with accessible references, styling, and improved scrolling.

IMPORTANT - UI CHANGE DEMONSTRATION

Screenshots:

[ben-fultz]

I looked into it and the remark-gfm plugin we're using can already handle footnotes, do you think we could use that instead of needing to add and maintain a custom remark plugin?

[thomapapavramidis]

I looked into it and the remark-gfm plugin we're using can already handle footnotes, do you think we could use that instead of needing to add and maintain a custom remark plugin?

Yeah good point. The issue I had observed was incorrect rendering for consecutive footnote definitions (the second definition appeared as literal markdown), and I added a regression test for that. But I agree I should avoid adding maintenance burden if remark-gfm can cover this directly (which it definitely can). If you want I’ll refactor to use remark-gfm and keep only the styling improvements.

[ben-fultz]

Yeah think that would be the best long term, thanks.