Add simple recursion examples: factorial, fibonacci, sum_of_digits

[Toshaksha]

Describe your change:

• Add an algorithm (simple recursion examples for beginners)

This pull request adds a new file: simple_recursion.py under the other/ directory.
It includes beginner-friendly recursive functions:

• factorial(n)
• fibonacci(n)
• sum_of_digits(n)

Each function includes:

• Python type hints
• Proper docstrings with doctests
• Basic error handling

External references:

• https://en.wikipedia.org/wiki/Factorial
• https://en.wikipedia.org/wiki/Fibonacci_number
• https://en.wikipedia.org/wiki/Digital_root

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• No issue is being resolved in this PR, so the issue-closing keyword is not included..