enables `mermaid` in docs (#935)

Short and sweet PR that addresses the follow-up in #909: enabling `mermaid` on our docs site, but making it clear that users still need to opt-in to use it. I've also added an example in-action.

To test: [see the Netlify deploy](https://deploy-preview-935--just-the-docs.netlify.app/docs/ui-components/code/#mermaid-diagram-code-blocks).