gitweb: Add gitweb.conf(5) manpage for gitweb configuration files

gitweb: Add gitweb.conf(5) manpage for gitweb configuration files

Much of what is in gitweb.conf.txt has been pulled directly from the
README file of gitweb.  The manpage was supplemented with description
of missing gitweb config variables, and with description of gitweb's
%features.

There remains a bit of redundancy, which should be reduced if
possible... but I think some of duplication of information is
inevitable.

[jn: Improved, extended, removed duplicate info from README]

Signed-off-by: Drew Northup <drew.northup@maine.edu>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>