docs: pps.txt: convert to ReST and rename to pps.rst

This file is already in a good shape: just its title and
adding some literal block markups is needed for it to be
part of the document.

While it has a small chapter with sysfs stuff, most of
the document is focused on driver development.

As it describes a kernel API, move it to the driver-api
directory.

In order to avoid conflicts, let's add an :orphan: tag
to it, to be removed when added to the driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>