Add detailed API documentation, error handling, and security best pra…

Created by: Krugou

…ctices to backend/README.md.

• API Documentation

  • Add detailed descriptions of each endpoint under Admin Routes.

• Error Handling

  • Add sections for custom error types, error handling in routes, controllers, and models.

• Security Best Practices

  • Add sections for input validation and sanitization, authentication and authorization, rate limiting, and logging.

Add DOCUMENTATION.md with project architecture, design patterns, and coding standards.

Update .github/copilot-instructions.md to reference new documentation files.