Docs/mm/damon/design: add a section for the relation between Core and Modules layer

Add overall desription of the interface and the relation between the Core
and the Modules layer under 'Overall Architecture' section.

Link: https://lkml.kernel.org/r/20230525214314.5204-7-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>