add docs/opendtu_swagger_openapi.yaml

[stefan123t]

implements #378

@tbnobody you may also add it to the docs repo and build an interactive API documentation
and/or keep it in the code repo docs/ folder too

[tbnobody]

Thank you.... based on your idea I've already added a lot of api endpoint stuff to the documentation.
(see https://www.opendtu.solar/firmware/web_api/)

[stefan123t]

This is awesome, thanks for taking the initiative !