docs: move pack 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 various documentation pertaining to the *.pack format and related
files, and updating things that refer to it to link to the new
location.

By moving these we can properly link from the newly created
gitformat-commit-graph to a gitformat-chunk-format page.

Integrating "Documentation/technical/bitmap-format.txt" and
"Documentation/technical/cruft-packs.txt" might logically be part of
this change, but as those cover parts of the wider "pack
format" (including associated files) that's documented outside of
"Documentation/technical/pack-format.txt" let's leave those for now,
subsequent commit(s) will address those.

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

Documentation/howto/recover-corrupted-object-harder.txt

@@ -68,7 +68,7 @@ Note that the "object" file isn't fit for feeding straight to zlib; it
 has the git packed object header, which is variable-length. We want to
 strip that off so we can start playing with the zlib data directly. You
 can either work your way through it manually (the format is described in
-link:../technical/pack-format.html[Documentation/technical/pack-format.txt]),
+linkgit:gitformat-pack[5]),
 or you can walk through it in a debugger. I did the latter, creating a
 valid pack like: