AFAIK there is no documentation beside the code and its comments, but this in combination with a look into the API calls from the frontend is comparably easy to understand.
These are the basic API URLs:

• https://api.utopian.io/api/posts
• https://api.utopian.io/api/users
• https://api.utopian.io/api/stats
• https://api.utopian.io/api/moderators
• https://api.utopian.io/api/sponsors
  for the options of each call, see the according file in src/server/models.