Request: make md_document() support TOC (table of content)

[TimTeaFan]

I'm using {hugodown} to render blog posts on my website. With longer blog posts it would be great to have the option to include a table of content (TOC), which is, to my understanding, not possible with hugodown::md_document().

I thought a viable workaround would be to replace hugodown::md_document() with rmarkdown::md_document(). However, this led to rendering problems.

I then took the relevant code from rmarkdown::md_document() and included it in my forked version of {hugodown} in the toc branch.

I do understand that {hugodown} is focused on a streamlined, highly minimal HTML output and adding a TOC might contradict this philosophy - in this case please close this issue. Otherwise I'd happily PR the branch.