docs: move cruft pack docs to gitformat-pack

docs: move cruft pack docs to gitformat-pack

Integrate the cruft packs documentation initially added in
3d89a8c1180 (Documentation/technical: add cruft-packs.txt, 2022-05-20)
to the newly created "gitformat-pack" documentation.

Like the "bitmap-format" added before it in
0d4455a3ab0 (documentation: add documentation for the bitmap format,
2013-11-14) the "cruft-packs" were documented in their own file.

As the diff move detection will show there is no change to
"Documentation/technical/cruft-packs.txt" here except to move it, and
to "indent" the existing sections by adding an extra "=" to them.

We could similarly convert the "bitmap-format.txt", but let's leave it
for now due to a conflict with the in-flight ac/bitmap-lookup-table
series.

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