feat: add kth next greater element script to /data_structures/stacks/.

[StarsExpress]

Describe your change:

Add kth Next Greatest Element (NGE) algorithm using k monotonic decreasing stacks.
It generalizes the existing first NGE to an arbitrary k, running in O(kn) average time with O(n) space.

• 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".

[algorithms-keeper]

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Repository:

• Contributing guidelines
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• Formatted string literals (f-strings)
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• doctest
• unittest
• pytest

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[StarsExpress]

@poyea
This is ready to be merged. Please have a look~
If you have any questions, feel free to ask me😄

[StarsExpress]

@cclauss
@MaximSmolskiy

I submitted this pull request about two weeks ago. Could you please have a look?

Feel free to ask me anything 🙏
Thank you very much~

[Copilot]

Pull request overview

Adds a new stack-based algorithm implementation to compute the k-th Next Greater Element (NGE) for every element in an input array, generalizing the existing 1st-NGE approach.

Changes:

• Introduces find_kth_next_greater_element() using k monotonic stacks to compute k-th NGE results.
• Adds doctest examples and a __main__ block for local doctest execution + simple timing.

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[StarsExpress]

@poyea
Included inside my newest commit are all suggested changes from you and Copilot.
All tests have passed. Please have a look. Thank you for all your feedbacks.