Add examples for pure functions and destructuring defaults

[mohandand]

This PR adds two new best-practice examples to the guide:

1. Pure Functions vs. Side Effects (in the Functions section)

   • Demonstrates the importance of avoiding side effects
   • Includes good vs. bad examples with clear reasoning

2. Destructuring Defaults & Nested Destructuring (in the Objects and Data Structures section)

   • Shows how to provide safe defaults and avoid runtime errors
   • Includes clean parameter handling with modern syntax

[Imran-imtiaz48]

This section provides a clear and concise explanation of the difference between pure functions and side effects, with practical examples to illustrate both good and bad practices. The examples are especially effective in demonstrating why pure functions are preferable—highlighting predictability, testability, and ease of refactoring. The "bad" code block effectively showcases common pitfalls like mutating external state, while the "good" example demonstrates clean and functional code by returning new values instead of modifying existing ones. The added inline comments are helpful for reinforcing key points. Overall, this is a solid addition to your documentation or GitHub README—well-structured, informative, and easy to understand.