Mermaid is great for simple stuff. If you want a more advanced approach where you can even have multiple levels. Lool at c4. The best way to do c4 without paying a dime I think is https://likec4.dev/
But again, for simple stuff, mermaid! It's also natively supported in a lot of markdown renderers, like gitlab
Indeed they do. It works but it is not great. I tried it for a couple of projects but eventually switched away from it because the layout renderer is pretty bad at this time and the results are not aesthetically pleasing at all, at times being difficult to read because the layout algorithm is so bad.
There's also PlantUML (lots of ads on the page, unfortunately) with support for all sorts of diagram types, and it's extensible. Swimlane, UML, sequence and about 20 more.
LikeC4 looks really cool. I spent a little time with the documentation and it seems like it requires that it runs its own server to render the diagram. In other words no export to html or svg. Do you know if that's the case?
But again, for simple stuff, mermaid! It's also natively supported in a lot of markdown renderers, like gitlab