I’ve released a tool for authoring UNIX manual pages using a markdown-ish source format:
Ron is a humane text format and toolchain for creating UNIX man pages, and things that appear as man pages from a distance. Use it to build and install standard UNIX roff man pages or to generate nicely formatted HTML manual pages for the web.
It still needs some work but can produce useful output for both roff and HTML. The sources are on GitHub.