Documentation: Document the NVMe PCI endpoint target driver

Add a documentation file
(Documentation/nvme/nvme-pci-endpoint-target.rst) for the new NVMe PCI
endpoint target driver. This provides an overview of the driver
requirements, capabilities and limitations. A user guide describing how
to setup a NVMe PCI endpoint device using this driver is also provided.

This document is made accessible also from the PCI endpoint
documentation using a link. Furthermore, since the existing nvme
documentation was not accessible from the top documentation index, an
index file is added to Documentation/nvme and this index listed as
"NVMe Subsystem" in the "Storage interfaces" section of the subsystem
API index.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>

Documentation/PCI/endpoint/index.rst

@@ -15,6 +15,7 @@ PCI Endpoint Framework
    pci-ntb-howto
    pci-vntb-function
    pci-vntb-howto
+   pci-nvme-function
 
    function/binding/pci-test
    function/binding/pci-ntb

Documentation/PCI/endpoint/pci-nvme-function.rst

@@ -0,0 +1,13 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+PCI NVMe Function
+=================
+
+:Author: Damien Le Moal <dlemoal@kernel.org>
+
+The PCI NVMe endpoint function implements a PCI NVMe controller using the NVMe
+subsystem target core code. The driver for this function resides with the NVMe
+subsystem as drivers/nvme/target/nvmet-pciep.c.
+
+See Documentation/nvme/nvme-pci-endpoint-target.rst for more details.

Documentation/nvme/index.rst

@@ -0,0 +1,12 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+NVMe Subsystem
+==============
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:
+
+   feature-and-quirk-policy
+   nvme-pci-endpoint-target