[petepete]

PlantUML diagrams are generated from plain text, they can easily be embedded in markdown or even in code (I'm sure there's a plug-in for that!)

Plenty of examples here.

https://plantuml.com/state-diagram

[gregmac]

And importantly, because they're text, they can live in a version-controlled environment. I mainly use them within Confluence, and even there you can diff between versions to see changes.