Add Euclidean Algorithm for GCD

Create euclidean.js file in Math_Algorithms directory.

Describe Euclidean Algorithm. This algorithm repeatedly replaces the larger number by its remainder when divided by the smaller number until the remainder is zero.

Implement the Euclidean Algorithm to find the greatest common divisor (GCD) of two numbers.

Provide a detailed explanation of the algorithm and its time complexity.

Add the algorithm to the corresponding section in the README.

Create Math_Algorithms/euclidean.js and start by reviewing the surrounding algorithm files for their export and documentation conventions. Implement GCD using the repeated remainder process, document the algorithm and its time complexity, and add it to the corresponding README section; done means the new file and README entry are present and explain the behavior clearly.

Written by the indexing model from the issue text.