Mermaid support

[garrett]

As GitHub has Mermaid support for issues, pull requests, and rendered Markdown (on the site), we might want to support Mermaid in-page on the website as well.

The best way to do this would be in-browser, client-side, so we can preserve the GitHub Pages ease of use, including forked previews.

https://github.com/mermaid-js/mermaid/blob/develop/docs/usage.md#using-mermaid has instructions.

We'd want to include a mermaid.js inside our website repo (not use a CDN) and point it to rendered code blocks. I'm not sure if we can do that directly via the JS and/or have to do some special handling in Jekyll templates.

It's probably mostly straightforward?