Add maths/factorial.c — simple factorial program in C

[srujanasadhusharma-source]

Description of Change

Added a simple C program maths/factorial.c that computes the factorial
of a non-negative integer using a for loop, as I'm a beginner here.

The program:

• Prompts the user for an integer.
• Calculates n! using a loop.
• Displays the result.

Author: Nagaraj (https://github.com/srujanasadhusharma-source)

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• I acknowledge that all my contributions will be made under the project's license.

Added maths/factorial.c program — beginner-friendly factorial implementation in C.

[srujanasadhusharma-source]

Hello maintainers

This is my first contribution to TheAlgorithms/C.
I’ve added a simple C program maths/factorial.c that computes the factorial of a number using a loop.
Please review my PR and let me know if any changes are needed.
Thank you for maintaining this amazing project!