docs-rst: Add a new directory for PCI documentation

Add a new directory in the driver API guide for PCI-specific documentation.

This is in preparation for adding a new PCI P2P DMA driver writers guide
which will go in this directory.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Sanyog Kale <sanyog.r.kale@intel.com>
Cc: Sagar Dharia <sdharia@codeaurora.org>

Documentation/driver-api/index.rst

@@ -29,7 +29,7 @@ available subsections can be seen below.
    iio/index
    input
    usb/index
-   pci
+   pci/index
    spi
    i2c
    hsi

Documentation/driver-api/pci/index.rst

@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+The Linux PCI driver implementer's API guide
+============================================
+
+.. class:: toc-title
+
+	   Table of contents
+
+.. toctree::
+   :maxdepth: 2
+
+   pci
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`