[DOCUMENTATION] Setup OpenAPI Documentation

### User Story
As a developer, I want automatically generated documentation for REST endpoints so I can easily understand what each endpoint does.

### Technical Tasks
- Follow [this tutorial](https://idratherbewriting.com/learnapidoc/openapi_tutorial.html)
- You don't need to follow it word for word
- Make sure you can explain the process of adding endpoints to the documentation, so we can get devs to do this in the future

### Acceptance Criteria
- an endpoint returns API docs as a readable page
- another endpoint returns the OpenAPI/Swagger definition in JSON