docs/admin-guide/mm: add high level concepts overview

The are terms that seem obvious to the mm developers, but may be somewhat
obscure for, say, less involved readers.

The concepts overview can be seen as an "extended glossary" that introduces
such terms to the readers of the kernel documentation.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>