chore: implement mermaid.js

[Jcahilltorre]

Proposed changes

Adds the mermaid script to the theme.

Documentation: https://mermaid.js.org/intro/

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[ADubhlaoich]

LGTM. It would make sense to socialize this among the product teams, though I'm not sure if it'll be used for a 1:1 replacement of some existing diagrams.