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>

Documentation/admin-guide/mm/index.rst

@@ -15,12 +15,17 @@ are described in Documentation/sysctl/vm.txt and in `man 5 proc`_.
 
 .. _man 5 proc: http://man7.org/linux/man-pages/man5/proc.5.html
 
+Linux memory management has its own jargon and if you are not yet
+familiar with it, consider reading
+:ref:`Documentation/admin-guide/mm/concepts.rst <mm_concepts>`.
+
 Here we document in detail how to interact with various mechanisms in
 the Linux memory management.
 
 .. toctree::
    :maxdepth: 1
 
+   concepts
    hugetlbpage
    idle_page_tracking
    ksm