Add openapi.json so it can be viewed by OCS-API Viewer app

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Is your feature request related to a problem? Please describe.
When using external automation tools or if you are scripting knowing API endpoints is very important. Currently, there is a quite old API documentation at https://deck.readthedocs.io/en/latest/API/ but e.g. Talk relies on the OCS-API Viewer app for Nextcloud, always displaying the latest API docs. Deck isn't available there.

Describe the solution you'd like
Publish an openapi.json which can be read by the OCS-API Viewer app.

Describe alternatives you've considered
Since I can't find any openapi.json file I can't even use other viewers. Now it's try and error with the old documentation.

Additional context

Start by comparing Deck's available API endpoints with the existing API documentation at deck.readthedocs.io and the requirements of the OCS-API Viewer app. Done means an openapi.json is published for Deck and can be consumed by the viewer and other OpenAPI tools.