Makefile: add quick-install-doc for installing pre-built manpages

Makefile: add quick-install-doc for installing pre-built manpages

This adds and uses the install-doc-quick.sh file to
Documentation/, which is usable for people who track either the
'html' or 'man' heads in Junio's repository (prefixed with
'origin/' if cloned locally).  You may override this by
specifying DOC_REF in the make environment or in config.mak.

GZ may also be set in the environment (or config.mak) if you
wish to gzip the documentation after installing it.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>