gitweb: Add gitweb(1) manpage for gitweb itself

gitweb: Add gitweb(1) manpage for gitweb itself

Most of what is in gitweb.txt it has been pulled directly from the
README and INSTALL files of gitweb.

Current version is somewhat based on structure of SVN::Web manpage
(one of web interfaces for Subversion).

gitweb.conf(5) i.e. gitweb configuration manpage now refers to
appropriate sections in gitweb(1).  gitweb/README now refers to
gitweb/INSTALL and gitweb(1) manpage.  gitweb/INSTALL now refers to
gitweb.conf(5) and gitweb(1).

Inspired-by: Drew Northup <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>