Add comprehensive documentation with usage guides and examples for all features

[Copilot]

This PR addresses the request for general usage documentation by providing a comprehensive documentation suite for WeihanLi.EntityFramework.

What's Added

📚 Complete Documentation Structure

• Enhanced README.md - Clear feature overview, quick start example, and navigation
• Getting Started Guide (docs/GettingStarted.md) - Step-by-step tutorial for new users
• Complete Usage Guide (docs/Usage.md) - Comprehensive examples for all features
• Advanced Features Guide (docs/AdvancedFeatures.md) - Performance optimization and integration patterns

🎯 Features Documented

The documentation now covers all major features with practical examples:

• Repository Pattern - Basic CRUD, advanced queries, bulk operations, pagination
• Unit of Work - Transaction management, multi-repository operations, error handling
• Audit System - Complete setup, custom stores, property enrichment, querying audit records
• Soft Delete - Entity configuration, operations, restoration, bulk soft delete
• Auto-Update - Timestamp management, user tracking, custom auto-update logic
• Database Extensions - JSON functions, bulk updates, query helpers, table/column names
• Performance Optimization - Projections, caching, streaming, connection pooling
• Testing Strategies - Unit testing with mocks, integration testing, test configurations
• Advanced Patterns - Custom interceptors, CQRS, event-driven architecture

📖 Example Code Coverage

Each feature includes multiple working examples:

// Repository with auto-update and audit
public async Task<Product> CreateProductAsync(string name, decimal price)
{
    var product = new Product { Name = name, Price = price };
    return await _repository.InsertAsync(product);
    // CreatedAt/UpdatedAt automatically set, audit record created
}

// Unit of Work for complex transactions
public async Task ProcessOrderAsync(Order order, List<OrderItem> items)
{
    var orderRepo = _unitOfWork.GetRepository<Order>();
    var itemRepo = _unitOfWork.GetRepository<OrderItem>();
    
    var createdOrder = await orderRepo.InsertAsync(order);
    foreach (var item in items)
    {
        item.OrderId = createdOrder.Id;
        await itemRepo.InsertAsync(item);
    }
    
    await _unitOfWork.CommitAsync(); // Single transaction
}

🔧 Practical Benefits

• Copy-paste ready examples for immediate implementation
• Troubleshooting section covering common issues and solutions
• Best practices for performance and maintainability
• Cross-linked navigation between documentation sections
• Version compatibility guidance for different EF Core versions

The documentation transforms the library from "mainly using the sample" to having comprehensive, searchable documentation that covers every feature with real-world examples.

Fixes #74.

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[WeihanLi]

LGTM overall