MAINTAINERS/MEMORY MANAGEMENT: add document files for mm

Memory managment subsystem documents ('admin-guide/mm/' and 'mm/' under
'Documentation/') are not marked as managed under memory management
subsystem.  This makes 'get_maintainer.pl' for changes to the documents
sub-optimal.  Mark the documents as part of mm subsystem on MAINTAINERS
file.

Link: https://lkml.kernel.org/r/20241101202311.53935-1-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>