doc: Rewrite manpage with scdoc

[tmccombs]

I'm not entirely sure if this is worth it.

On the one hand, I think the source is clearer, easier to read, and easier to modify.

However, scdoc has a few limitations and odd edge cases, that makes it a little awkward to work with.

Another alternative is we could use ASCIIDOC with pandoc or asciidoctor to convert it.