I think my solution here is not the best way to use Mermaid in Hugo.
What is Mermaid? Mermaid is a sequence diagram generator written in Javascript.
https://github.com/mermaid-js/mermaid
Not only sequence diagrams, Mermaid can also generate simple graphs.
Enabling Mermaid in Hugo - use mermaid-cli Failures I tried two patterns but I failed both cases.
Make a template in the theme Mainroad. I’ve Tried to copy MathJax template but failed. But this is the simplest and useful, so I need to study Hugo more.