Improve selection sort doctests and documentation

[23f3003167]

• Add detailed algorithm description explaining the approach
• Document time complexity O(n²) and space complexity O(1)
• Add 11 comprehensive doctest examples covering:
  • Basic cases (already covered)
  • Edge cases (single element, empty list)
  • Sorted/reverse-sorted inputs
  • Lists with duplicates
  • Verification against standard library sorted() function

These improvements help new learners understand the algorithm better and verify correctness across diverse inputs.

Describe your change:

Improve selection sort documentation and doctests in one algorithm file.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[23f3003167]

Looking forward to feedback. Happy to make any requested changes quickly. Thanks for your time reviewing.