Add comprehensive DDD/CQRS documentation with core concepts and advanced patterns

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This PR adds extensive documentation covering Domain-Driven Design (DDD), Command Query Responsibility Segregation (CQRS), and Event Sourcing concepts as implemented in this boilerplate.

What's Added

📚 Core DDD Documentation

• DDD Objectives and Reasoning - Why use DDD, benefits, and when to apply it
• Bounded Context - Context boundaries with practical User domain examples
• Value Objects vs Entities - Core building blocks with Email/User comparisons
• Aggregates and Aggregate Root - User aggregate as consistency boundary with business rules
• Layered Architecture - Clean architecture with dependency inversion and layer enforcement
• Anti-Corruption Layer - Protection patterns with repository adapters and translation examples

🚌 CQRS and Event Sourcing Documentation

• CQRS Overview - Command/Query separation with performance and scalability benefits
• Command Bus - Write operations, middleware, error handling with Symfony Messenger
• Event Sourcing - Event streams, snapshots, temporal queries, and versioning strategies
• Application Workflow - Complete request flow from UI through all architectural layers
• Symfony Messenger Integration - Configuration, routing, async processing, and testing

🚀 Advanced Topics

• Testing Strategies - Comprehensive testing approach for Unit, Integration, and E2E testing in DDD/CQRS applications
• Read and Write Models - Optimization strategies, projections, and storage patterns
• Event Sourcing Patterns - Advanced patterns including versioning, process managers (sagas), and monitoring

Key Features

Practical Examples: All concepts include real code examples from this boilerplate's User domain, showing exactly how theoretical concepts are implemented in practice.

Complete Implementation Guide: Demonstrates how DDD patterns work together - from domain aggregates through command handlers to event projections.

Production-Ready Patterns: Advanced topics cover scaling, monitoring, error handling, and evolution strategies for real-world applications.

Testing Guidance: Detailed testing strategies for each architectural layer, including test data builders, custom assertions, and async operation testing.

Updated Structure

The main README now provides clear navigation with categorized documentation sections:

• 📚 Core Concepts (DDD and CQRS fundamentals)
• 🚀 Getting Started (practical implementation guides)
• 🔧 Advanced Topics (production patterns and testing)
• 🛠️ Development Setup (tooling and deployment)

This documentation serves as a comprehensive learning resource for developers wanting to understand and implement DDD/CQRS patterns with Symfony, with all examples drawn from the actual boilerplate implementation.

Fixes #192.

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