Over the last month I have been using C4 PlantUML extensively to design a system.
PlanrUML is a set of tools that can be used to generate UML diagrams. In particular I have been using the C4 Model combined with Activity diagrams.
These can be used to generate diagrams that can be included in the github project and referenced from the README,md file.
Both vscode and intellij have decent plugins that can allow for rapid previews.
You can keep multiple diagrams source in a single file which helps to keep them synchronized in version control.
Developer Rants 