[feature]: Support for MermaidJs diagrams

Is there an existing issue for this?

• I have searched the existing issues

Summary

It would be great if Pages and Issues could support [MermaidJs diagrams]( in their main content areas.

As far as I know it's simply a case of including their JS file, and it can also be lazy loaded so it does not impact users not using the feature.

Why should this be worked on?

This would allow diagrams as code to be easily inserted into Pages or Issues. Diagrams as code would save on disk space used by the image based equivalent, and are much more suitable for tracking in version control etc.

Mermaid seems to be one of the mainstream accepted formats. Gitlab and Github support Mermaid diagrams.

If Mermaid is a no-go, then PlantUML is also a good option but it requires a Java program to render the diagrams, whereas Mermaid is pure JavaScript.