Create silhouette_score.cpp

[Swish78]

Description of Change

Added an implementation of the Silhouette Score calculation in C++ following TheAlgorithms project guidelines. The Silhouette Score is a metric used to assess the quality of clustering results. The code includes functions to calculate the Silhouette Score for a dataset based on pairwise distances between data points and their cluster labels. Three test cases have been added to validate the correctness of the implementation.

Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, tests pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments are changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes: This PR adds the Silhouette Score calculation code following project guidelines, including tests and documentation.