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 <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Mauro Carvalho Chehab <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Vinod Koul <[e-mail]>
Cc: Linus Walleij <[e-mail]>
Cc: Logan Gunthorpe <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Cc: Sanyog Kale <[e-mail]>
Cc: Sagar Dharia <[e-mail]>

Documentation/driver-api/index.rst

···
    iio/index
    input
    usb/index
-   pci
+   pci/index
    spi
    i2c
    hsi

Documentation/driver-api/pci/index.rst

···
+.. 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`