You should check out Scribble which uses Racket. It has a bunch of different renderers including HTML and Markdown. However, unlike Markdown, you can easily write extensions for it in Lisp, even within the document itself. https://docs.racket-lang.org/scribble/running.html#%28part._...

I suppose that makes it inappropriate for something like rendering comments, but it's a great way to write documents. I used it a lot for writing papers since there's a lot less boilerplate and was much easier to use than TeX.