docs: leds: fix the following docu build warning

Add leds-qcom-lpg to Documentation/leds/index.rst to fix the following
doc build warning.

Documentation/leds/leds-qcom-lpg.rst: WARNING: document isn't included in any toctree

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230125154426.12464-1-skhan@linuxfoundation.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>