docs: add complexity analysis to insertion_sort docstring

[Harman-2]

Describe your change:

• Documentation change?

Added a clear Complexity Analysis section to the insertion_sort function docstring inside sorts/insertion_sort.py.

It documents:

• Time Complexity: Best Case $O(n)$, Average Case $O(n^2)$, and Worst Case $O(n^2)$.
• Space Complexity: $O(1)$ auxiliary space.

Fixes #14866

Checklist:

• I have read CONTRIBUTING.md.
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• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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• 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".