docs: nvdimm: add it to the driver-api book

The descriptions here are from Kernel driver's PoV.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>

Documentation/driver-api/index.rst

@@ -44,6 +44,7 @@ available subsections can be seen below.
    mtdnand
    miscellaneous
    mei/index
+   nvdimm/index
    w1
    rapidio/index
    s390-drivers

Documentation/driver-api/nvdimm/index.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 ===================================
 Non-Volatile Memory Device (NVDIMM)
 ===================================

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.rst for more details.
+	  Documentation/driver-api/nvdimm/nvdimm.rst for more details.
 
 	  Say Y if you want to use an NVDIMM