Enable websocket API doc generation

[joffrey-bion]

We should find a way to generate a documentation of the websocket API to make it easier to maintain.

Swagger seemed to be an option, but is fundamentally tied to REST, and thus not adequate to use for websockets as-is:
swagger-api/swagger-socket#47
OAI/OpenAPI-Specification#55

JsonDoc looks like a viable option, but we should first have a closer look at how it works.

[joffrey-bion]

Made obsolete now that the whole project is 100% Kotlin.
Local project dependencies allow to have even better than documentation: code completion and type safety.