docs/admin-guide: introduce basic index for mm documentation

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

Documentation/admin-guide/index.rst

@@ -63,6 +63,7 @@ configure specific aspects of kernel behavior to your liking.
    pm/index
    thunderbolt
    LSM/index
+   mm/index
 
 .. only::  subproject and html
 

Documentation/admin-guide/mm/index.rst

+=================
+Memory Management
+=================
+
+Linux memory management subsystem is responsible, as the name implies,
+for managing the memory in the system. This includes implemnetation of
+virtual memory and demand paging, memory allocation both for kernel
+internal structures and user space programms, mapping of files into
+processes address space and many other cool things.
+
+Linux memory management is a complex system with many configurable
+settings. Most of these settings are available via ``/proc``
+filesystem and can be quired and adjusted using ``sysctl``. These APIs
+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
+
+Here we document in detail how to interact with various mechanisms in
+the Linux memory management.