docs: move commit-graph format docs to man section 5

docs: move commit-graph format 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 documentation for the commit-graph format into man
section 5 and the new "developerinterfaces" category. This change is
split from subsequent commits due to the relatively large amount of
ASCIIDOC formatting changes that are required.

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