Add comprehensive SOLID principles documentation with TypeScript, Python, and Angular examples

[Copilot]

This PR adds a comprehensive guide to SOLID principles in software development, following the same high-quality format as the existing DRY-Principle.md documentation.

What's Added

Complete SOLID Principles Documentation (SOLID-Principles.md)

• Detailed explanations of all five SOLID principles (SRP, OCP, LSP, ISP, DIP)
• Historical context covering Robert C. Martin's contribution and why these principles were needed
• Dual language examples with both TypeScript and Python code demonstrating each principle
• Bad vs Good code comparisons showing principle violations and proper implementations
• Real-world GitHub projects that enforce SOLID principles in their codebases
• Best practices and common anti-patterns to avoid

Angular Task Management Example (angular-solid-example/)

A complete Angular project demonstrating SOLID principles in practice:

• Task management application with proper separation of concerns
• Service interfaces following Interface Segregation Principle
• Dependency injection configuration showcasing Dependency Inversion Principle
• Modular architecture with clear examples of each SOLID principle
• Comprehensive comments showing bad code examples as educational contrasts
• Complete UI implementation with HTML templates and CSS styling

Key Features

The documentation covers all requested aspects:

1. Detailed explanations of what SOLID principles are and why they matter
2. Creator information - Robert C. Martin (Uncle Bob) and the historical context
3. Necessity explanation - Problems with traditional OOP design that SOLID solves
4. Code examples in both TypeScript and Python for each principle
5. Angular project demonstrating real-world application with refactoring examples
6. GitHub projects like Angular Framework, Spring Framework, ASP.NET Core that enforce SOLID

Educational Value

Each principle includes:

• Clear definitions and explanations
• Why it matters for software design
• Practical code examples showing violations and proper implementations
• Real-world applications and benefits

The Angular example serves as a practical tutorial showing how to refactor code to follow SOLID principles, making it an excellent learning resource for developers.

Fixes #63.

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