refactor: Add comprehensive documentation with examples to charging-engine modules

- Add module-level documentation to lib.rs with usage example
- Document Claims and AuthConfig structs with examples in auth.rs
- Add ChargingEngine struct documentation with initialization example
- Document ChargingRule, SubscriberAccount, AccountStatus, UsageEvent, UsageType, and RatingPlan with examples in types.rs
- Add CircuitState, CircuitBreakerError, and CircuitBreaker documentation with examples in circuit_breaker.rs

apps/charging-engine/src/lib.rs

@@ -1,3 +1,37 @@
+//! # Charging Engine
+//!
+//! A high-performance telecom charging engine for real-time usage-based billing.
+//!
+//! ## Features
+//!
+//! - Real-time rating and billing
+//! - Circuit breaker for fault tolerance
+//! - Rate limiting for API protection
+//! - Comprehensive monitoring and metrics
+//!
+//! ## Example
+//!
+//! ```rust,no_run
+//! use charging_engine::ChargingEngine;
+//! use charging_engine::charging::types::UsageEvent;
+//!
+//! #[tokio::main]
+//! async fn main() -> Result<(), Box<dyn std::error::Error>> {
+//!     let engine = ChargingEngine::new("redis://localhost").await?;
+//!
+//!     let event = UsageEvent {
+//!         imsi: "1234567890".to_string(),
+//!         volume: 1000,
+//!         timestamp: chrono::Utc::now(),
+//!     };
+//!
+//!     let cost = engine.calculate_cost(&event).await?;
+//!     println!("Cost: {}", cost);
+//!
+//!     Ok(())
+//! }
+//! ```
+
 pub mod auth;
 pub mod charging;
 pub mod circuit_breaker;