Generate and publish OpenAPI documentation for Explorer Users

[omad]

It would be great if an Explorer instance could publish it's own documentation on the API endpoints it provides.

This should be generated based on the actual code, and published as an OpenAPI specification.

Ideally, as well as the OpenAPI spec, there should be a nice UI provided by something like with ReDoc or Swagger UI.

At the very least we need the STAC API endpoints documented, but there's probably a few others should be included too.

There are a multitude of libraries for adding this to Flask, I'm not sure which to pick. Maybe flasgger which is popular, or flask-smorest.

We could also work out how hard a conversion from Flask to FastAPI would be, since it includes built in OpenAPI/UI support, as well as being new and fast and cool. There's probably some nasty gotchas to beware of though, so not something to undertake lightly.