Add swap two numbers

[princesharma2004]

Description of Change

Added a program to swap two numbers using reference variables in C++.
The program includes:

• A function swap_two_numbers(int &a, int &b) to swap two integers.
• Two test functions (test1() and test2()) to verify the correctness of the swap function.
• Assert statements for automatic validation.
• Doxygen-style comments for documentation and better understanding.
• A main() function that runs the tests and prints Success. on passing all assertions.

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Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is 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.

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Notes:
Added a fully tested and documented implementation of swapping two numbers using a temporary variable. Code is designed to be self-explanatory and follows Doxygen documentation style.