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 <[e-mail]>
Cc: Steven Royer <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/index.rst

···
    filesystems/index
    vm/index
    bpf/index
+   misc-devices/index
 
 Architecture-specific documentation
 -----------------------------------

Documentation/misc-devices/ibmvmc.rst

···
 .. SPDX-License-Identifier: GPL-2.0+
+
 ======================================================
 IBM Virtual Management Channel Kernel Driver (IBMVMC)
 ======================================================

Documentation/misc-devices/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+Assorted Miscellaneous Devices Documentation
+============================================
+
+This documentation contains information for assorted devices that do not
+fit into other categories.
+
+.. class:: toc-title
+
+	   Table of contents
+
+.. toctree::
+   :maxdepth: 2
+
+   ibmvmc