Documentation of Use Cases and Reasons Behind API and Data fields

Created by: hermannsblum

During the discussion about #117 it occurred to us that a lot of previous discussions got forgotten about why we introduced certain api fields.
It is therefore necessary that we document the reason and function behind every api field in 2 ways:

1. the public documentation of api fields -> /docs
2. the user and/or admin documentation for more technical background and all data fields

However, we need to agree on a format for this