custom markdoc component for footnotes

[lschierer]

It appears that this theme does not implement markdoc the same way that I would get if I ran

npx astro add markdoc

in a generic astro project, and that as a result some of the documentation at the Astro doesn't fully apply. As the todo list is locked, I cannot comment there. I'm trying to follow the advice given at
#70 to add footnote support, as that is not supported in the upstream Markdoc. I suspect that would be useful to more than just me, and might be something you want to consider for inclusion in the theme if it can be made to work.