Add an OpenAPI file to describe the API

[alexgarel]

That would enable:

• documenting the API
• making it easily usable by other applications

See also: openfoodfacts/openfoodfacts-server#8325

[john-gom]

I'm not sure I'd like to document the current API as it's really more of a fudge to make it look like MongoDB. I separately did a POC on creating a GraphQL API so that might be a way forward for more external access