doc: pm: Add reference for device runtime pm
doc: pm: Add reference for device runtime pm

Add a way to reference device runtime documentation from
other documents.

Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr