docs(parser): clarify expiration validation behavior in PasetoParser

[rrrodzilla]

Summary

Adds explicit documentation that PasetoParser::default() automatically validates expiration and not-before claims, addressing user confusion reported in issue #26.

Changes

• README: Added new "Validating tokens and handling errors" section with practical example
• src/lib.rs: Updated parser documentation with automatic validation details
• src/prelude/paseto_parser.rs: Enhanced struct-level and method documentation

Key Documentation Additions

1. Default behavior clarification: PasetoParser::default() validates exp and nbf claims automatically
2. Alternative constructor: PasetoParser::new() creates parser without automatic time-based validations
3. Error handling example: Demonstrates token validation with expired token error handling
4. Return value documentation: Explicitly documents GenericParserError return for validation failures

Example Added

use rusty_paseto::prelude::*;

let key = PasetoSymmetricKey::<V4, Local>::from(Key::from(b"wubbalubbadubdubwubbalubbadubdub"));

// Create a token that expires in the past
let token = PasetoBuilder::<V4, Local>::default()
  .set_claim(ExpirationClaim::try_from("2019-01-01T00:00:00+00:00")?)
  .build(&key)?;

// Parsing the expired token returns an error
match PasetoParser::<V4, Local>::default().parse(&token, &key) {
  Ok(json_value) => {
    println!("Token valid: {}", json_value);
  }
  Err(err) => {
    // This will print: "This token is expired"
    eprintln!("Token validation failed: {}", err);
  }
}

Testing

• Verified code compiles with cargo build --no-default-features --features v4_local
• Checked clippy passes with zero warnings: cargo clippy --all-targets -- -D warnings
• Formatted code with cargo fmt

Closes #26