PCI/P2PDMA: Add P2P DMA driver writer's documentation

Add a restructured text file describing how to write drivers with support
for P2P DMA transactions.  The document describes how to use the APIs that
were added in the previous few commits.

Also adds an index for the PCI documentation tree even though this is the
only PCI document that has been converted to restructured text at this
time.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>