Documentation improvements

[TobiasKaestnerLumira]

Despite the high quality of existing documentation in Zephyr and Bridle there are still some gaps that are worth being looked at to see if we could come up with a solution that would also be of value for upstream.

As a first example the .cmake modules have grown to significant complexity yet lack any representation within the documentation, ie you have to read the .cmake files themselves to understand what's going on

some other projects exist that try to introduce a cmake domain for Sphinx, e.g. http://docs.ros.org/en/api/catkin/html/dev_guide/generated_cmake_api.html
https://gitlab.com/Pro1/doxygen-cmake-sphinx
https://github.com/slurps-mad-rips/moderncmakedomain