Documentation: add ibmvmc to toctree(index) and fix warnings

Fix Sphinx warnings in ibmvmc.rst, add an index.rst file in
Documentation/misc-devices/, and insert that index file into the
top-level index file.

Documentation/misc-devices/ibmvmc.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
Documentation/misc-devices/ibmvmc.rst:: WARNING: document isn't included in any toctree

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Steven Royer <seroyer@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>