Add documentation rules / definition

[MrGeneration]

In order to help contributors and also other members, we need to have a common rule set for a global goal.
This should help to reduce discussions where to find what and if something is really required to add / provide.

[ralf401]

@MrGeneration I'll clean the docu repos a bit.

Regarding this issue: There is already a section for that in place in the new docs and an issue for extending it.