Add Shell Sort algorithm (fixes #13887)

[rachit-27]

Summary

Implements the Shell Sort algorithm inside sorts/shell_sort.py.

Details

• Added Shell Sort function with proper docstring, type hints, and doctests.
• Created corresponding unit tests under tests/sorts/test_shell_sort.py.
• Verified functionality using pytest — all tests passed locally.
• Code follows repository conventions and naming patterns.

Related Issue

Fixes #13887