docs: add comprehensive rustdoc for testing module

[joshrotenberg]

Summary

Adds detailed module-level documentation for the testing utilities with examples covering all features and common patterns.

Also includes a comprehensive example file (examples/testing_utilities.rs) that serves as both runnable tests and documentation.

Closes #224

Documentation Includes

Why Use This?

• Automatic cleanup benefits
• Panic-safe behavior
• Debug-friendly options
• Network support overview
• Ready checks explanation

Configuration Options

• Lifecycle Control: stop_on_drop, remove_on_drop
• Debugging Failed Tests: keep_on_panic, capture_logs
• Ready Checks: wait_for_ready (automatic or manual)
• Container Reuse: reuse_if_running for faster local dev
• Network Support: with_network, remove_network_on_drop
• Fast Cleanup: stop_timeout configuration

Multi-Container Tests

• ContainerGuardSet usage with shared networks
• Coordinated startup and cleanup

Accessing Container Information

• connection_string()
• template()
• container_id()
• logs()
• is_running()

Common Patterns

• Reusable test fixtures
• Unique container names for parallel tests
• Manual cleanup when needed

Feature Flag Requirements

• Shows how to enable the testing feature in Cargo.toml

Example File

Added examples/testing_utilities.rs with 10 runnable tests demonstrating:

• Basic ContainerGuard usage with automatic cleanup
• Connection strings for service access
• Configuration options (stop timeout, debug mode, reuse)
• ContainerGuardSet for multi-container tests
• Container info access (logs, IDs)
• Test fixture patterns

Run with: cargo test --example testing_utilities --features testing

Preview

The documentation renders beautifully in rustdoc with proper code highlighting and linked type references.