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>