Add tutorial-style documentation: rcm(7)

Add a manpage with a tutorial, named `rcm`. This covers how to get
started from nothing, how to convert an existing dotfiles directory, and
why to use suite at all.
Re-write the manpages in mdoc

Convert all the manpages to mdoc. This gives us access to the mdoc suite
of tools, which includes HTML conversion, plus mdoc is a more
expressable and natural format in general.