docs: power: convert docs to ReST and rename to *.rst

Convert the PM documents to ReST, in order to allow them to
build with Sphinx.

The conversion is actually:
  - add blank lines and indentation in order to identify paragraphs;
  - fix tables markups;
  - add some lists markups;
  - mark literal blocks;
  - adjust title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>

Documentation/driver-api/usb/power-management.rst

@@ -46,7 +46,7 @@ device is turned off while the system as a whole remains running, we
 call it a "dynamic suspend" (also known as a "runtime suspend" or
 "selective suspend").  This document concentrates mostly on how
 dynamic PM is implemented in the USB subsystem, although system PM is
-covered to some extent (see ``Documentation/power/*.txt`` for more
+covered to some extent (see ``Documentation/power/*.rst`` for more
 information about system PM).
 
 System PM support is present only if the kernel was built with