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!

[prashantpiyush1111]

Reviewed the PR. The change is appropriately scoped to backtracking/all_permutations.py and adds a clear docstring plus basic doctests for both list and string inputs. The proposed examples match the function's printed output, and there are no unrelated changes. I don't see a blocking issue in the shown diff.

One thing to note: the PR body says pytest was run locally, but that execution is not independently verifiable from the PR diff alone. Please ensure the relevant test suite/doctests pass in CI before merge.

[dwaddle]

@prashantpiyush1111 Thank you for the review! I can confirm that all automated CI checks
│ (including the build and linting checks) have successfully passed on this PR.