Message overview image is incomplete

[mpkorstanje]

🤔 What's the problem you've observed?

The image in the message overview in the readme is incomplete. This is bad because the structure is an important part of the message protocol and helps people find their way.

Additionally, by being a PNG it is hard to edit and will go out of date.

✨ Do you have a proposal for making it better?

Replace all images with mermaid diagrams or plant uml. I believe GitHub supports both.

📚 Any additional context?

The purpose of the diagrams is to show the relationships between different message. Not the details of the indivual messages. So ideally we coverall messages that go in the envelope and select parts of the Gherkin document.