docs: move http-protocol docs to man section 5

docs: move http-protocol docs to man section 5

Continue the move of existing Documentation/technical/* protocol and
file-format documentation into our main documentation space by moving
the http-protocol.txt documentation over. I'm renaming it to
"protocol-http" to be consistent with other things in the new
gitformat-protocol-* namespace.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>