mailmap doc: create a new "gitmailmap(5)" man page

mailmap doc: create a new "gitmailmap(5)" man page

Create a gitmailmap(5) page similar to how .gitmodules and .gitignore
have their own pages at gitmodules(5) and gitignore(5). Now instead of
"check-mailmap", "blame" and "shortlog" documentation including the
description of the format we link to one canonical place.

This makes things easier for readers, since in our manpage or
web-based[1] output it's not clear that the "MAPPING AUTHORS" sections
aren't subtly different, as opposed to just included.

1. https://git-scm.com/docs/git-check-mailmap

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