idea: add swagger ui

[hejjoe]

see also
https://github.com/swagger-api/swagger-ui
https://search.dip.bundestag.de/api/v1/swagger-ui/

[tifa365]

I've build an API specification upon the OpenAPI standard, but it would need another testing.

https://tifa365.github.io/bt-to-tests/

I suppose the fact that it's not working on my fork is only a CORS issue, but in general there are no major problems to implement this. Would be nice to get in touch to talk about this.