Improve insertion_sort docstring with complexity analysis

[shreya-learn-explores]

Describe your change:

This pull request improves the docstring of the insertion_sort() function in sorts/insertion_sort.py by adding time and space complexity analysis.

Changes made:

• Added Best Case time complexity: O(n)
• Added Average Case time complexity: O(n²)
• Added Worst Case time complexity: O(n²)
• Added Space Complexity: O(1) (in-place sorting)

This helps improve code readability and makes the algorithm easier to understand for beginners.

Fixes #14866

• 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.
• [x ] 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 Improve docstrings in insertion_sort.py to include complexity analysis #14866".

[zain-cs]

Reviewed both commits and the final diff.

Code correctness:
The complexity claims added to the docstring are accurate against the actual implementation:

• Best Case: O(n) — correct, since the inner while-loop exits immediately on an already-sorted run.
• Average/Worst Case: O(n²) — correct, confirmed by the shift-back behavior in the while-loop on reverse-sorted input.
• Space Complexity: O(1) — correct, sorting is done in place with no auxiliary collection.

CI status:

• Initial commit (7513a1b) had 2/5 checks passing.
• pre-commit.ci's auto-fix commit (9e3cd71) brought it to 4/5, resolving lint/formatting issues.
• deploy_docs shows as skipped, which is expected behavior on PR branches (it only runs on merge to master).
• build, sphinx/build_docs, and ruff all pass on the final commit.

Scope:
Documentation-only change (+8/−0), no logic touched, existing doctests unaffected.

LGTM — approving.