Document the implementation concept of the of the Companion App

For former "Remote settings app" is now becoming "Companion App". Initially, the app was a part of the prototype to try our the use of Qt Remote Objects as a transport for APIs auto-generated from QFace. That prototype was very successful and fit well with many needs in Qt Auto. Among other aspects, the "Remote settings app" can serve as a reference implementation of a companion app which connects to the central ECU running the IVI and can access values via network which can be shown and changed on a remote device via network. Thanks Qt being cross-platform, and thanks a few design approaches in the existing "Remote settings app", this provides a complete app running the same way on any desktop or mobile device supporting Qt 5.12 and later.

A developer should understand the software architecture aspects applied to the implementation of the "Remote settings app". Reviewing of its code does not seem completely sufficient. This should be extended by a dedicated page in the docs as well as comments in the code itself if required.