docs: add practical example of state management with dependency injec…

[ArslanYousaf12]

Enhanced Dependency Management Documentation with Practical Examples

What's being changed

Added a comprehensive practical example to the dependency management documentation that demonstrates the integration of state management with dependency injection. This helps developers understand how to properly structure their GetX applications with real-world patterns.

Before: Documentation only covered basic dependency injection concepts and methods.
After: Added practical example showing how to:

• Create and manage controllers with GetX
• Implement proper dependency injection with services
• Handle reactive state management
• Structure widget integration

Documentation Updates

• Added new section "Practical Examples" in dependency_management.md
• Added fully documented code example with detailed comments
• Included explanation of best practices

Pre-launch Checklist

• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or @jonataslaw said the PR is test-exempt.
• All existing and new tests are passing.

Note: This PR is documentation-only and updates the existing dependency management guide with practical examples. No code changes were made that would require additional tests.