docs: Improve VS Code JUnit tutorial for better clarity and student usability

[arshinsikka]

Summary

This PR enhances the “VS Code - Testing with JUnit” tutorial to make it more beginner-friendly and improve the overall student experience.

Key Improvements

• Added an overview box outlining learning outcomes
• Mentioned required VS Code extensions (Java Pack, Test Runner)
• Added step to verify setup via Testing icon
• Included screenshot of ideal folder structure in VS Code
• Moved SE book link earlier for better visibility
• Clarified how to run tests using green play button

These changes aim to reduce confusion and ensure smooth onboarding for first-time users. Feedback welcome!

Closes #149

[damithc]

@arshinsikka Thanks for the PR.
@YosieSYX (and others) for your inputs.
I think this is a good time to look at all three JUnit tutorials (the general one, VSCode one, and IDEA one) and consolidate so that there is no duplication (repeating the same content using <include .../> directive is still fine though -- just don't copy paste the same content).
The general JUnit tutorial should continue to exist, as there are students not using IDEs.

[damithc]

@YosieSYX, any inputs?