refactor: add memoized fibonacci example

[ahmed-abohmaid]

Eng. Brian Holt explained that we can optimize the Fibonacci calculation using memoization. When analyzing the recursive calls for fibonacci(5), we noticed that certain calculations, like fibonacci(3), are repeated multiple times. By memoizing these results, we can avoid recalculating them in subsequent recursive calls, improving the performance of the function.

This version added the example of the memiozed fibonacci function.