Add Shell Sort Algorithm

[Gauri2882]

Describe your change:

• [x] Add an algorithm
• [ ] Fix a bug or typo in an existing algorithm
• [ ] Add or change doctests?
• [ ] Documentation change

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Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, I opened a separate PR for this algorithm only.
• [x] All new Python files are placed inside an existing directory (sorts/).
• [x] All filenames are in all lowercase characters with no spaces or dashes (shell_sort.py).
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [x] This algorithm includes a reference link to Wikipedia (Shell Sort explanation).
• [ ] Fixes #ISSUE-NUMBER (optional — only check if you linked an issue)