Add API documentation

[darkobits]

Adding documentation for the API would help development and testing of its consumers, most notably the Signal Stickers website. The primary driver of this request is to better handle errors returned by the API. Specifically, what HTTP status codes each route/method can return, and the expected error payload for each. It looks like in most cases we use a payload like:

{
  "error": "This is an API error message."
}

However, knowing when to expect a JSON payload with an error for a non-200 response would make error-handling code on the front-end easier to manage.

Additionally, having documentation would let us write type definitions for expected request and response payloads so that our API interactions are fully type-safe.