Improve comments, add doctests for kahns_algorithm_topo.py

[Hardvan]

The improvements to the code include adding a more detailed docstring that explains the purpose, parameters, return values, and the use of Kahn's Algorithm for topological sorting. The docstring now includes doctests, providing example usage and handling edge cases such as graphs with cycles. Inline comments clarify key steps like indegree calculation, queue initialization, and the BFS process. Additionally, explicit handling and explanation of cycle detection were added, making the code more readable and understandable while ensuring that it is easy to validate using the provided test cases.

• 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".

[tianyizheng02]

LGTM, thanks for your contribution!

[Hardvan]

Thanks!