Add more information about the purpose of each key in the JSON spec

[SchoolGuy]

Currenty we have an empty table when trying to describe what purpose the keys in our JSON model have. i now found with reading the cobbler/cobbler code at least the purpose of the signatures key. Additionally we have the description of some of the keys already in the docstring of the code of our llibrary.

Purpose of the signatures is the following: It represents the portion of the path from the root of the repository which may indentify the os version. This must be always one or more paths relative from the root of the ISO. If no values in this array are present the attribute should be considered invalid.

[SchoolGuy]

Context in Cobbler: https://github.com/cobbler/cobbler/blob/b68e37be693975fc0e6bbc41f55696fbba829b14/cobbler/modules/managers/import_signatures.py#L196-L237

[SchoolGuy]

This issue has gotten more priority since I was asked by people in the community multiple times what the functionality of the keys is actually doing and I didn't have any reference at hand.