docs: nvdimm: convert to ReST

Rename the nvdimm documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>

Documentation/nvdimm/index.rst

+:orphan:
+
+===================================
+Non-Volatile Memory Device (NVDIMM)
+===================================
+
+.. toctree::
+   :maxdepth: 1
+
+   nvdimm
+   btt
+   security

Documentation/nvdimm/security.txt → Documentation/nvdimm/security.rst

-NVDIMM SECURITY
+===============
+NVDIMM Security
 ===============
 
 1. Introduction
@@ -138,4 +139,5 @@ This command is only available when the master security is enabled, indicated
 by the extended security status.
 
 [1]: http://pmem.io/documents/NVDIMM_DSM_Interface-V1.8.pdf
+
 [2]: http://www.t13.org/documents/UploadedDocuments/docs2006/e05179r4-ACS-SecurityClarifications.pdf

drivers/nvdimm/Kconfig

@@ -33,7 +33,7 @@ config BLK_DEV_PMEM
 	  Documentation/admin-guide/kernel-parameters.rst).  This driver converts
 	  these persistent memory ranges into block devices that are
 	  capable of DAX (direct-access) file system mappings.  See
-	  Documentation/nvdimm/nvdimm.txt for more details.
+	  Documentation/nvdimm/nvdimm.rst for more details.
 
 	  Say Y if you want to use an NVDIMM