While I acknowledge that this will lower the barrier to contribute to our manuals, shouldn't we wait until people came to an agreement in NixOS/rfcs#72 a majority is fine with?

The lowdown page states (in a footnote):

You may be tempted to write manpages in Markdown, but please don't: use mdoc(7), instead — it's built for that purpose! The man output is for technical documentation only (section 7).

Some more improvements:

• The nix command now has a manpage (though most of the subcommands don't have a description yet, just an automatically generated list of flags).

• The nix.conf manpage / manual section is now generated from the C++ sources. So we no longer have two sets of option descriptions.

• Similarly, the manual section that lists the builtin functions is generated from the C++ sources. You can now also do :doc builtins.<name> to get documentation about a builtin function (rendered using the lowdown library).

Unless there are major objections I'd like to merge this (without prejudice to whatever markup language / tools people end up using for Nixpkgs/NixOS). Having a manpage for nix and the :doc command in nix repl alone is worth merging this.