docs(sorts): add complexity analysis to bubble_sort.py docstrings

[ltianyi992]

Closes #14593

Description

Improves the docstrings in sorts/bubble_sort.py to include time and space complexity analysis, making the implementation more educational and consistent with the repository's goal as a learning resource.

Changes

bubble_sort_iterative:

• Added a brief description of how the algorithm works
• Added Time Complexity: O(n) best case (early exit), O(n²) average/worst
• Added Space Complexity: O(1) — in-place
• Added Stable: Yes

bubble_sort_recursive:

• Added a brief description of the recursive approach
• Added Time Complexity: O(n²) all cases (no early exit due to call stack)
• Added Space Complexity: O(n) — recursive call stack
• Added Stable: Yes

Checklist

• I have read CONTRIBUTING.md
• This PR only changes one file
• Documentation change only
• All existing doctests pass (python3 -m doctest sorts/bubble_sort.py — 40 tests, all pass)