doc: guides: pm: device_runtime: new guide

Add a new and more extensive guide that describes the device runtime
API. It contains a brief introduction, API design principles and goals,
some example sequence diagrams and usage guidelines.

While the API still needs important additions, such a guide will
hopefully guide developers when trying to deal with device PM in Zephyr.

Note: sequence diagrams have been created using PlantUML (2021 version).
The rendered SVG diagrams embed the original code if they have to be
edited and re-rendered.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>