allow installation of man and html doc from the man and html branches

allow installation of man and html doc from the man and html branches

This patch introduces a make target "quick-install-html" which installs
the html documentation from the branch origin/html, without the need for
asciidoc/xmlto. This is analogous to the existing "quick-install-doc"
target for the man pages.

We advertise these targets in the INSTALL file now.

Signed-off-by: Michael J Gruber <michaeljgruber+gmane@fastmail.fm>
Signed-off-by: Junio C Hamano <gitster@pobox.com>