Add math snippets for factorial and Fibonacci calculations

[dibyam-jalan27]

Description

Type of Change

• ✨ New snippet
• 🛠 Improvement to an existing snippet
• 🐞 Bug fix
• 📖 Documentation update
• 🔧 Other (please describe):

Checklist

• I have tested my code and verified it works as expected.
• My code follows the style and contribution guidelines of this project.
• Comments are added where necessary for clarity.
• Documentation has been updated (if applicable).
• There are no new warnings or errors from my changes.

Related Issues

Closes #

Additional Context

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[Mathys-Gasnier]

Hey, we recently updated how snippets are stored, please update your PR to match the changes, you will be able to see how to add snippets in CONTRIBUTING.md.
For ref: #87

[danovdenys]

@dibyam-jalan27 It's likely the most inefficient way to calculate fibonacci sequence with O(2^N) time complexity and O(N) auxiliary space.
It is better to use loops or matrix exponentiation