Add graph images to our docs?

[augustearth]

Particularly on index.md, some graph images may help explain/advertise Carnival features.

[augustearth]

Mermaid diagrams can now be added to GitHub docs:

• https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams
• https://mermaid-js.github.io/mermaid/#/n00b-syntaxReference

[augustearth]

Mermaid works for markdown files displayed on GitHub.com, but not on Github pages at least not yet. Will add mermaid graphs where it seems appropriate.