Recursion Concepts in Python in Lesson 2

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Issue In the Function.py and functions.md file, there's alreasy a primer on creating and calling a function. But besides this, maybe a short refresher on recursion might help too

Proposed Pull Request Added a short primer on recursion using the classic example of calculating factorials. Deliberately didn't include hadnling edge cases like floating point numbers and mentioned it as a sort of 'challenge' in the functions.md file