Add practical usage examples

[RAprogramm]

Closes #5

Changes

Added four comprehensive examples demonstrating practical usage of cstring-array:

examples/basic.rs

• Basic CStringArray creation from Vec
• Accessing array properties (length, emptiness, pointer)
• Safe iteration over elements
• Individual element access
• Demonstrates automatic memory cleanup

examples/command_line.rs

• Real-world command-line argument handling
• FFI interoperability with C functions
• Simulates passing argv to C code
• Command parsing demonstration
• Shows proper argc/argv pattern

examples/env_vars.rs

• Creating arrays from environment variables
• KEY=VALUE formatting for C-style environment
• Filtering and transforming data
• Searching for specific variables
• Demonstrates use case for execve() and similar functions

examples/zero_copy.rs

• Zero-copy construction from Vec
• Performance comparison with regular construction
• When to use from_cstrings() vs new()
• Large dataset demonstration (1000 elements)
• Memory efficiency explanation

Features

• All examples compile without warnings
• Executable via cargo run --example
• Comprehensive inline documentation
• Real-world use cases
• REUSE 3.3 compliant
• Follow project code style guidelines

Testing

• All examples build successfully
• basic.rs runs and produces expected output
• command_line.rs handles arguments correctly
• env_vars.rs processes environment variables
• zero_copy.rs demonstrates performance benefits
• REUSE compliance verified

Documentation

Each example includes:

• File-level documentation explaining purpose
• Inline comments for clarity
• Usage instructions
• Real-world application scenarios

Run examples with:

cargo run --example basic
cargo run --example command_line -- --verbose file.txt
cargo run --example env_vars
cargo run --example zero_copy