docs, fix: fit fibonacci_sum.cpp to contributing guidelines #2456

[farhan900720-web]

Description of Change

Fixes #2456

Refactored math/fibonacci_sum.cpp to align with the repository's updated contribution guidelines. Specific changes include:

• Replaced single-letter variables with descriptive snake_case names.
• Added standard Doxygen comments to all functions.
• Included Time and Space complexity analysis for each algorithm.
• Replaced magic numbers with standard const variables.

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: Refactoring legacy code for the umbrella documentation and code quality issue.