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 <changbin.du@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/PCI/index.rst

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

Documentation/index.rst

@@ -101,6 +101,7 @@ needed).
    filesystems/index
    vm/index
    bpf/index
+   PCI/index
    misc-devices/index
 
 Architecture-specific documentation