Use help2man to generate interim man page

[casey]

Eventually, I'd like to have a voluminous man page that includes:

• Documentation of command line syntax (autogenerated by clap)
• The contents of the current readme (generated using asciidoctor's roff backend)
• The just file grammar

However, this is waiting on a few things, mostly for me to get to it, and for clap to land man page generation support.

In the mean time, @NickeZ who's working on packaging just for Debian, mentioned that we could use the help2man utility to generate a serviceable man page from the output of just --help.

This seems like a good interim step. In order to make sure that the man page is consistent across packages, we should build it and check it in to the repository as man/just.1