It may be even simpler: plain markdown files that explain architecture including C4 diagrams generated from text using plantuml (automated via invoke/fabric/whatever). Plantuml includes C4 in stdlib https://github.com/plantuml-stdlib/C4-PlantUML
As far as plantuml/graphviz/mermaid/similar UML tools, it's really cool stuff, I agree. My issue is that those tools are mainly built to render static diagrams (output to svg, png, etc.) Some eg mermaid are perhaps extensible for interactivity, but it's a little wonky. I think a lot of the value of whiteboard comes from the interactivity -- being able to click on a node to see the underlying code, being able to go through a guided walkthrough of a given flow, etc.
But using their UML authoring format is certainly worth looking into, since the LLMs are already so well-versed in it. i'll put some thought into this!