Documentation: PCI: convert pcieaer-howto.txt to reST

Convert plain text documentation to reStructuredText format and add it to
Sphinx TOC tree.  No essential content change.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/PCI/index.rst

@@ -14,3 +14,4 @@ Linux PCI Bus Subsystem
    msi-howto
    acpi-info
    pci-error-recovery
+   pcieaer-howto