Use lowdown to build manpages

Pandoc is a fantastic application. It's wonderful. I use it all the
time to write papers in markdown and have them converted to beautiful
LaTeX-driven PDFs. I have nothing against pandoc.

However, using it to make manpages is like driving a Mack truck down the
driveway to check the mailbox. On my system (8-core all-SSD on ZFS) it
takes nearly 6 hours to build pandoc, and also these days it doesn't
work (see PR below).

Instead, use lowdown to generate the manpages. They don't come out
exactly right, but it's close enough to be usable.

PR: 237878