doc: drop author/documentation sections from most pages

The point of these sections is generally to:

  1. Give credit where it is due.

  2. Give the reader an idea of where to ask questions or
     file bug reports.

But they don't do a good job of either case. For (1), they
are out of date and incomplete. A much more accurate answer
can be gotten through shortlog or blame.  For (2), the
correct contact point is generally git@vger, and even if you
wanted to cc the contact point, the out-of-date and
incomplete fields mean you're likely sending to somebody
useless.

So let's drop the fields entirely from all manpages except
git(1) itself. We already point people to the mailing list
for bug reports there, and we can update the Authors section
to give credit to the major contributors and point to
shortlog and blame for more information.

Each page has a "This is part of git" footer, so people can
follow that to the main git manpage.