Mermaid.js support

[brownsarahm]

We've been looking for good ways to include diagrams and I think mermaid is a good solution now.

I cannot figure out how to determine if it is supported or how to support it in the workbench, I think this is related to #115 conceptually. I know how to dig through a python based system for its dependencies and I have had trouble figuring out the packaging strucutre and dependencies for this.

For example, I knew for incorporating mermaid into jupyterbook, I needed to look for a sphinx extension that supported mermaid.

[zkamvar]

Luckily, support in The Workbench can be added by updating the HTML headers (see the MathJax implementation) and JavaScript in {varnish}: carpentries/varnish#34

The only downside is that the diagrams will not show offline, only the code block.