Enhance middleware documentation for Azu framework

- Expanded the built-in middleware section to provide a comprehensive overview of available handlers, including detailed usage examples and configuration options.
- Improved the custom middleware documentation, outlining the interface, common patterns, and advanced techniques for implementing middleware.
- Enhanced error handling middleware documentation, detailing error types, handling strategies, and integration with external error tracking services.
- Added best practices for middleware implementation, focusing on clarity, performance, and maintainability.

These updates aim to improve developer understanding and usability of middleware in Azu, aligning with the framework's emphasis on type safety and performance.

Author: eliasjpr