Document API with OpenAPI

[boldt]

Let's create a OpenAPI documentation for the API. This should be tool-based, so it will be created automatically from the source code. I did that some yrs ago, but probably there are are better/easier ways today:

For example:

• https://blog.logrocket.com/documenting-your-express-api-with-swagger/

[opatut]

Changed the title and your original description to use current terminology -- swagger is just a tool now, the spec is called "OpenAPI"