Include CONTRIBUTING.md into the generated docs

[JOJ0]

but at the same time keep it as a plain MD formatted file in the root of the git repo, which fits with the (quasi) standard we see in other git (github?) repos.

Motivation and thoughts: When I look at our generated docs at https://synadm.readthedocs.io (https://synadm.rtfd.io), it always feels like developers documentation is entirely missing. We are not even linking to it. In my opinion one well formatted documentation "entry point" is a goal. It would also support the current efforts to simplify README.md as much as possible. We can always link to the generated Sphinx docs from there. The same goes for the elsewhere discussed TROUBLESHOOTING.md, I suggest to also incorporate this into the generated docs and link to it from wherever we see fit.