convert plantUML to mermaid

[bclaremar]

Björn seems to need help to convert at

• https://github.com/UPPMAX/programming_formalisms/blob/main/docs/introductions/sdlc_tools.md

[richelbilderbeek]

Done!

@bclaremar: sometimes I needed to change things to make it fit better. I hope I made the right choices there :-)