doc: separate docs for creating a complete new template from adapting to a scenario

[Blesmol]

Currently there is an (outdated) documentation on how to write your own templates. Which is good and necessary, if it wasn't outdated. But ok, we will come to this.

But there should be a separate documentation only targeting the scenario where a user wants to adapt an existing chronicle template to a new scenario. The user probably doesn't need to know about the complete format. Only like "create file there, enter this, check whether everything fits, if not that is how you correct things".