I’ve recently gotten reacquainted with Doxygen, and it allows you to embed PlantUML source right in your source code or Markdown files. Easy to write, easy to update, and stored as plaintext right in your source tree. I don’t love Doxygen, but it’s doing a great job at what I’m trying to do (document a complex C++ project)
I use PlantUML in org-mode as well! That's actually where I started, but found it much easier to recruit other people to write their own documentation (with sequence and state diagrams) by not requiring them to use Emacs :)
