Adhere to the OpenAPI Specification (OAS)

[u-ways]

This ticket is to make our REST API follow the OpenAPI Specification (OAS)
To do that you will need to investigate, what parts of the OAS we are not following and then fix that accordingly.

Useful links:

• Our REST Spec: https://docs.google.com/spreadsheets/d/1-hm7sD0OtxqGTUjn38hMWFXHAkG-eP3Um2NbXiAXDNY/edit#gid=37578465
• Our REST doc: https://documenter.getpostman.com/view/13397421/TVeneoY3#intro
• OAS doc: https://oai.github.io/Documentation/start-here.html