[FEA] Add mermaid.js support

Describe the solution you'd like

I want to be able to write Mermaid diagrams in our docs, similar to what we can do in Github.

@jameslamb found this guide, which seems simple enough, but I don't know if there are hidden dragons. https://stuff-things.net/2025/01/19/mermaid-diagramming-in-jekyll-in-2025/

Describe alternatives you've considered
Static images of diagrams created elsewhere. This is viable, but the perk of mermaid diagrams is that the source code of the diagram lives in the docs and can be fixed or improved in the future.

Additional context

• https://github.com/mermaid-js/mermaid
• https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams

Start by reading the linked guide on Mermaid diagrams in Jekyll and inspecting this repository's documentation rendering and build configuration. Done means Mermaid source can be written in the docs and renders as diagrams without breaking the existing documentation build.