docs: add time and space complexity to selection_sort docstring

[kl2400033266]

Added Time Complexity O(n^2) and Space Complexity O(1) documentation to the selection_sort function docstring, improving code documentation for beginners as requested in issue #13948.

Describe your change:

This PR adds time and space complexity information to the selection_sort function docstring in sorts/selection_sort.py. The documentation now includes:

• Time Complexity: O(n^2) - with explanation of the nested loops
• Space Complexity: O(1) - noting that only constant extra space is used

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

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, please open separate PRs for separate algorithms.

Related to issue #13948

[kl2400033266]

Hi @vishalbeniwal @mCoding 👋 Just checking in to see if this PR is ready for review. All checks have passed, and the documentation updates for selection_sort are complete. Whenever you have a moment, I would appreciate an approval so it can be merged. Thank you!