Added Combination Sum Problem in backtracking

[AnushaV01]

Description of Change

Added Combination Sum problem under backtracking/.
Implemented both brute-force and optimized backtracking approaches.
Included problem explanation, time and space complexity, and a working main() with example.

Checklist

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

Notes: This is my first contribution to this repository. Tried to make the solution educational and easy to follow for beginners.