Add a mermaid field.

Problem/Motivation

For documenting a site, it would be very helpful to have a field available to add to a Content Model Document that supports adding Mermaid diagrams.

Mermaid diagram
- Diagram Title (required)
- Diagram Caption (required)
- diagram (required)
- show copy-able mermaid code (optional)

This should use the same twig template / render array as the module already uses.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes