Create API documentation

[christianlupus]

Currently the API between frontend and backend just relies upon common sense. To simplify debugging, testing (#298), but also further development in general we should have a clear defined API. In fact, I found this issue during my checks on #362.

I suggest to create an OpenAPI definition for our app and commit that YAML file to the codebase.

This is of course related to #294.

[christianlupus]

This was solved in #757.