Docs/mm/damon/design: document DAMON sample modules

Docs/mm/damon/design: document DAMON sample modules

People sometimes get confused about the purposes of DAMON special-purpose
modules and sample modules.  Clarify those on the design document by
adding a section describing their existence and purposes.

Link: https://lkml.kernel.org/r/20260118180305.70023-4-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>