feat: add Depth first search

[adi776borate]

Description of Change

• Added a new function to count the number of paths in a graph.
• Implemented a test case to validate the functionality of the new function.
• Improved documentation for better understanding of the algorithm.

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.

Notes: The new function counts the number of unique paths between two nodes in a graph. It can be particularly useful in graph-related problems where pathfinding is necessary.