README: update ast diagram to v6 using Mermaid class diagram

[binhtran432k]

🤔 What's changed?

• Update the AST diagram in README using mermaid class diagram

⚡️ What's your motivation?

• Fixes README: Gherkin grammar diagram (AST) is missing Gherkin v6 changes (Rules, ...) #141

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

♻️ Anything particular you want feedback on?

GitHub supports Mermaid diagrams directly within Markdown using Markdown-like syntax, which is very helpful for future changes.

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[mpkorstanje]

Cheers! Looks good. Apologies for the late review.

Would you be interested in replacing the other images with Mermaid diagrams as well?