Add a section covering the API of DAMON core layer on the design document.

Link: https://lkml.kernel.org/r/20230525214314.5204-10-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>