Docs/mm/damon/design: add a section for kdamond and DAMON context

The design document is not explaining about the concept of kdamond and
the DAMON context, while usage document does.  Those concept explanation
should be in the design document, and usage document should link those.
Add a section for those.

Link: https://lkml.kernel.org/r/20230907022929.91361-7-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>