docs/vm: add index.rst and link MM documentation to top level index

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

Documentation/index.rst

@@ -45,7 +45,7 @@ the kernel interface as seen by application developers.
 .. toctree::
    :maxdepth: 2
 
-   userspace-api/index	      
+   userspace-api/index
 
 
 Introduction to kernel development
@@ -89,6 +89,7 @@ needed).
    sound/index
    crypto/index
    filesystems/index
+   vm/index
 
 Architecture-specific documentation
 -----------------------------------