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>

drivers/power/supply/power_supply_core.c

@@ -607,7 +607,7 @@ int power_supply_get_battery_info(struct power_supply *psy,
 
 	/* The property and field names below must correspond to elements
 	 * in enum power_supply_property. For reasoning, see
-	 * Documentation/power/power_supply_class.txt.
+	 * Documentation/power/power_supply_class.rst.
 	 */
 
 	of_property_read_u32(battery_np, "energy-full-design-microwatt-hours",