Add mermaid support

[clouserw]

Let's add support for https://mermaidjs.github.io/#/ so we can get easy versioning on our diagrams.

It looks like it's supported as a plugin, but I don't know if the docker container we're using supports plugins. 🤞

[clouserw]

I looked in to doing this with a Plugin, generating the images ahead of time, and saving them to disk, but I think it would be a lot simpler to just include the JS and have the browser interpret them on the fly. (I don't know if there are limitations to that method).

That method is outlined at https://stackoverflow.com/questions/53883747/how-to-make-github-pages-markdown-support-mermaid-diagram

[lmorchard]

It looks like PR #23 renders Mermaid charts client-side with a third-party JS lib via script tag & CDN - would it at all be feasible to host our own copy of it?