docs: fit factorial.cpp to contribution guidelines

[realstealthninja]

Description of Change

fixed documentation of factorial.cpp
added much needed tests
added an exception so that numbers larger than an unsigned 64 bit integer is not used as an argument

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: part of #2456

[Panquesito7]

Thanks! 🚀
Will be merging in a few days if there are no objections. In case I forget, feel free to ping me. 🙂

[realstealthninja]

@Panquesito7