portable code to generate man pages?

[Greg Troxel <gdt%lexort.com@localhost>]

I am part of bup upstream, which has manpage source in markdown.  The
build uses pandoc to convert to mandoc, and also to html.

That would be fine if pandoc were written in a language that had a
reasonable implementation, but it's the usual Haskell compiler written
in Haskell with a complicated bootstrap story, with the result that on
NetBSD it works on amd64 and aarch64 only.  Not even i386!

I would like to hear people's recommendation for a tool that can:

  - process easy-to-write text file to mandoc and html
    + preference for markdown
  - does not require anything nonportable
    (accepting that python doesn't work on vax)
  - is incrementally lightweight, in a build that already requires
    python

asciidoctor pulling in ruby is unappealing.

A lot of people use sphinx.   I see hints that it can create man pages.
Has anyone written or maintained man pages in sphinx, and how did that
go?

  https://www.sphinx-doc.org/en/master/man/sphinx-build.html
  https://www.sphinx-doc.org/en/master/usage/builders/index.html