docs: Add visual diagram explaining instance vs global (static) variable behavior [variable-scope-diagram]

🖼️ Overview:
Includes `InstanceVariableVsGlobalVariable.png` to visually differentiate how instance and static variables behave in Java.

📌 Key Concepts Illustrated:
- Instance Variables:
  - Unique to each object
  - Stored separately for every instance
- Static Variables:
  - Shared across all instances
  - Single memory allocation at class level

🎯 Purpose:
- Reinforce conceptual understanding from code examples
- Useful for presentations, documentation, or onboarding materials

✅ Enhances learning through:
- Side-by-side comparison of memory structure
- Clear visual of object-specific vs shared data

Signed-off-by: Somesh diwan <[email protected]>

Author: Someshdiwan