Improve typing, doctests, and docstrings in gray_code_sequence

[JunaidIRF]

Describe your change:

• Parameterized return types with modern Python type hints (list[int] and list[str] instead of raw list).
• Added doctest coverage for the base case gray_code(0).
• Corrected minor typographical errors in docstrings ("one times" -> "one time", "grey" -> "Gray code").

Checklist:

• I have read the CONTRIBUTING.md guide.
• This pull request adheres to the project guidelines.
• All doctests pass (pytest --doctest-modules bit_manipulation/gray_code_sequence.py).
• Ruff linter passes (uvx ruff check bit_manipulation/gray_code_sequence.py).