Add a mermaid field.

While working on this, it made more sense to add this as a separate module. The module Mermaid Diagram Field → is now used to make the field available and provides there rendering for all other mermaid diagrams present in the Content Model Documentation module.

This will go out with 1.0.19.

Mermaid Diagram Field is now a dependency. If using composer it should come in automatically, but it will need to be enabled and added to your configuration.