Add API documentation for SCXA web app

[ke4]

Our API is currently not documented. We need to add documentation to our API.
We already had some user query about it.

Alfonso and Upendra was working on this topic already and there is a PR (#215), but as far as I know that PR contains implementation using OpenAPI. I think that PR can be closed.
We would like to use another approach that merges OpenAPI with RestDocs.
Here is the URL how to do that with a combined approach: https://github.com/ePages-de/restdocs-api-spec