Merge branch 'mm-rst' into docs-next

Mike Rapoport says:

  These patches convert files in Documentation/vm to ReST format, add an
  initial index and link it to the top level documentation.

  There are no contents changes in the documentation, except few spelling
  fixes. The relatively large diffstat stems from the indentation and
  paragraph wrapping changes.

  I've tried to keep the formatting as consistent as possible, but I could
  miss some places that needed markup and add some markup where it was not
  necessary.

[jc: significant conflicts in vm/hmm.rst]

include/linux/hmm.h

@@ -16,7 +16,7 @@
 /*
  * Heterogeneous Memory Management (HMM)
  *
- * See Documentation/vm/hmm.txt for reasons and overview of what HMM is and it
+ * See Documentation/vm/hmm.rst for reasons and overview of what HMM is and it
  * is for. Here we focus on the HMM API description, with some explanation of
  * the underlying implementation.
  *