This commit adds extensive documentation comments to all Rust modules
in the src/ directory to improve the docs.rs documentation quality.

Changes:

• Add crate-level documentation to lib.rs with overview and examples
• Document tokenizer module: TokenizerBuilder and Tokenizer classes
• Document dictionary module: Dictionary, UserDictionary, and related functions
• Document metadata module: Metadata and CompressionAlgorithm types
• Document mode module: Mode enum and Penalty configuration
• Document schema module: Schema, FieldDefinition, and FieldType
• Document error module: LinderaError type
• Document util module: Python-Rust conversion utilities
• Document trainer module: model training and export functions
• Remove unnecessary #[allow(non_snake_case)] attribute from dictionary.rs

All documentation includes:

• Module-level descriptions and usage examples
• Detailed function/method documentation with arguments and return values
• Python code examples for end-user reference
• Error handling documentation where applicable