Documentation: add Linux PCI to Sphinx TOC tree

Add index.rst for PCI subsystem. More docs will be added later.

Signed-off-by: Changbin Du <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>

Documentation/PCI/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+Linux PCI Bus Subsystem
+=======================
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:

Documentation/index.rst

···
    filesystems/index
    vm/index
    bpf/index
+   PCI/index
    misc-devices/index
 
 Architecture-specific documentation