Experiment with PlantUML.
Using the plantUML server that links to a file in this repo. No need for secrets, keys, accounts, pipelines, just a link.
Copied example with file structure examples.
Changing the image might be an issue, because of caching.
Libraries available;
Code block with PlantUML. Adding a GitHub Action to generate the image; this is not needed using mermaid.js or on gitlab.
@startuml
Alice -> Bob: test
@enduml