[dev-v5][Docs] Add migration guide for @onclick

[MarvinKlein1508]

Pull Request

📖 Description

This PR adds a small migration text for @onclick to the FluentMenu migration. In v4 it was possible to use @onclick on the FluentMenuItem component. In v5 you can still define @onclick but it will result in a runtime exception. Found this one out during my migration progress - coul dbe helpful for others.

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps