added maximum subarray sum

[avin-madhu]

Describe your change:

• [x] Added the maximum subarray sum algorithm using Kadane's Algorithm in the array section of data structures.

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 have opened separate PRs for separate algorithms.
• [x] All new Python files are placed inside an existing directory.
• [x] All filenames are in lowercase characters with no spaces or dashes.
• [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] The new algorithm includes a URL that points to an external explanation (e.g., Wikipedia or similar).
• [x] If this pull request resolves one or more open issues, the description includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".