Docs/mm/damon/design: place execution model and data structures at the beginning

The execution model and data structures section at the end of the design
document is briefly explaining how DAMON works overall.  Knowing that
first may help better drawing the overall picture.  It may also help
better understanding following detailed sections.  Move it to the
beginning of the document.

Link: https://lkml.kernel.org/r/20231213190338.54146-4-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>