Docs/mm/damon/design: add sections for basic parts of DAMOS

DAMOS is an important part of DAMON, but the design doc is not covering
it.  Add sections for covering the basic part of DAMOS.

Link: https://lkml.kernel.org/r/20230525214314.5204-8-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>