mm,doc: Add new documentation structure

Closely following the outline of Mel Gorman's book "Understanding the
Linux Virtual Memory Manager", add a new outline.  Preserve the current
contents of the mm underneath the new outline so we can transition those
documents to a more sensible place later.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20220507134947.444287-1-willy@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>