docs: add docstring and doctests to generate_all_permutations)

[dwaddle]

This pull request adds a missing docstring to the generate_all_permutations
function in backtracking/all_permutations.py to improve code readability and
maintainability. It also includes two simple doctests to verify the correct
behavior of the function.

- [x] Added a docstring explaining the function's purpose
- [x] Added `doctests` for basic list and string inputs
- [x] Ran `pytest` locally and all tests passed successfully

This is my first contribution to this repository. Let me know if any changes are

required!