Docs/mm/damon/design: add a section for the modules layer

Add a section for covering DAMON modules layer to the design document.

Link: https://lkml.kernel.org/r/20230525214314.5204-11-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>