The design doc is missing overall picture of DAMON.  Add a section for
overall architeucture and layers.

Link: https://lkml.kernel.org/r/20230525214314.5204-4-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>