I'm currently doing an architecture decision record about Markdown documentation, and will add Markdoc to the candidates. The leaders so far are MDX 2 with plugins for JSX-style work, and Svelte for a fully dynamic site.
I'm aware of the Markdoc page about "Why not MDX?" which explains that Markdoc is deliberately less capable than MDX. But the page doesn't show how to do typical needs (IMHO) such as loops or substitutions. And for simple writing, compare with standard markdown annotated with tags/templates using Liquid or Jinja or similar?