manpage: Add separate manpages for each command

Using docker as a model, this update creates separate manpages
for each OSTree command, such that the main manpage is not
cluttered and the separate pages can provide more in-depth detail
and exanples that might be useful to a user.  Each page includes
synopsis, description, example, and a list of options if needd.
This update also alphabetizes the usage error output for ostree
and ostree admin so that it matches the list on the manpage.