sequencediagram.org is a pleasant (and free) tool for generating standalone diagrams like this outside of GitHub. I discovered it while helping Corrily with their docs, e.g. the diagram at the top of this page: https://docs.corrily.com/docs/price-optimization-overview
Something to know is that sequencediagram.org seems to be just a website skin around PlantUML Sequence Diagrams[0]. PlantUML is open source and is ultimately built on top of Graphviz as well.
Another nice thing is that PlantUML is supported (alongside Mermaid) in GitLab, and has been supported for at least two years. You can embed PlantUML diagrams into your README without committing any images to the repo or linking to external images, just like Mermaid.[1]
