docs: leds: add it to the driver-api book

The contents of leds driver docs is messy: it has lots of
admin-guide stuff and kernel internal ones, just like other
driver subsystems.

I'm opting to keep the dir at the same place and just add
a link to it. This makes clearer that this require changes.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/index.rst

@@ -91,6 +91,7 @@ needed).
 
    driver-api/index
    core-api/index
+   leds/index
    media/index
    networking/index
    input/index

Documentation/leds/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ====
 LEDs