Update site diagrams to use mermaid

[samspills]

trafficstars

https://typelevel.org/cats-effect/docs/core/io-local and https://typelevel.org/cats-effect/docs/std/supervisor (once the site redeploys) both include diagrams. Can we render these diagrams more nicely with mermaid?

[iRevive]

There is a mermaid usage example in otel4s docs: https://typelevel.org/otel4s/tracing-context-propagation.html https://github.com/typelevel/otel4s/blob/main/docs/tracing-context-propagation.md

Looks better indeed 🙂

[aluscent]

I'd like to pick this one. Is it contributable for contributors from outside?