Add practical usage examples

[RAprogramm]

Add real-world usage examples to demonstrate the library capabilities.

Objectives

• Create examples/ directory with practical examples
• Show common use cases for CStringArray
• Demonstrate FFI integration patterns
• Provide ready-to-run code samples

Proposed Examples

1. basic.rs - Simple string array creation and usage
2. exec.rs - Calling execve() with CStringArray
3. env.rs - Environment variables with CStringArray
4. ffi_integration.rs - Complex FFI scenario

Implementation

• Create examples/ directory
• Add runnable examples with cargo run --example
• Document each example with comments
• Add examples to README.md
• REUSE compliant headers

Benefits

• Easier onboarding for new users
• Clear real-world usage patterns
• Better documentation
• Shows library capabilities