Add openapi.yaml

[pavog]

This pull request adds the openapi.yaml file that describes the API.
The content is equivalent to the documentation in the README.md, but expressed in the OpenAPI 3.1.0 yaml format.

This allows the API definition to be used for for everything related to OpenAPI, such as:

• generating API clients in various programming languages with the OpenAPI Generator
• creating interactive API documentation (e.g. Swagger UI, Redoc, ...)
• and much more