Fix: Enhance unique function solution for better clarity and correctness

[SIMRAN-202]

This PR completes the unique function solution to improve clarity, correctness, and adherence to best practices.

Fixes - #3720

Key Changes:

• Added Explanation for solution.md file:

  • Added detailed explanations for each step to enhance understanding.
  • Organized the code structure and comments for better readability.

• Code Refinements:

  • Updated the logic to use the Set method for efficient uniqueness checking.
  • Simplified and optimized the code for better performance.

• Enhanced Documentation:

  • Added comments to explain each code block for better clarity.

PR Goals:

• Provide a complete and correct solution for filtering unique values from an array.
• Enhance the quality and usability of the solution for the JavaScript Info repository.

[CLAassistant]

All committers have signed the CLA.