plainbox:docs: add job unit man page

This patch moves documentation of job units from plainbox/docs/author/jobs.rst
to plainbox/docs/manpages/plainbox-job-units.rst. It also tweaks formatting of
the file to better suit manpage viewing.

Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge of lp:~kissiel/checkbox/job-unit-manpage/ by tarmac [r=zkrynicki][bug=][author=kissiel]"