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

Documentation/leds/index.rst

@@ -26,3 +26,4 @@ LEDs
    leds-lp55xx
    leds-mlxcpld
    leds-sc27xx
+   leds-qcom-lpg