docs: power: add it to to the main documentation index

The power docs are orphaned at the documentation body.

While it could likely be moved to be inside some guide, I'm opting to just
adding it to the main index.rst, removing the :orphan: and adding the SPDX
header.

The reason is similar to what it was done for other driver-specific
subsystems: the docs there contain a mix of Kernelspace, uAPI and
admin-guide. So, better to keep them on its own directory,
while the docs there are not properly classified.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/index.rst

@@ -111,6 +111,7 @@ needed).
    netlabel/index
    networking/index
    pcmcia/index
+   power/index
    target/index
    timers/index
    watchdog/index

Documentation/power/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ================
 Power Management