[orga] Provide script for "next free rule number"

[cgebken]

Context

Each guideline rule has an ID that is displayed in the UI:

These IDs are used in the URLs and to clearly reference rules.

When creating a new rule, contributors must add a rule ID at the very beginning of their file:

Currently, they do not know which ID to put / which ID is up next / is already used.
We would like to motivate people to contribute to our guidelines.
Thus, we should keep the effort to suggest a new rule as low as possible.

Goal

We should at least provide some minimal guidance.

•  Provide a script users can manually run to get the next free rule ID.
•  Add the script to the Contributing file.