Add Dutch National Flag sorting algorithm

[chan9an]

Implements the Dutch National Flag algorithm to sort an array of 0s, 1s, and 2s in-place. Includes error handling for invalid values and provides examples in the docstring.

Describe your change:

• 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.

File added: arrays/dutch_national_flag_sort.py
Reference: https://en.wikipedia.org/wiki/Dutch_national_flag_problem

[cclauss]

Do we need more Dutch National Flag sorts? It is not a very useful algorithm.
@priya-sundaram-dev

[priya-sundaram-dev]

@cclauss My recommendation: close this as a duplicate. master already ships sorts/dutch_national_flag_sort.py — a full dutch_national_flag_sort(sequence: list) -> list with doctests and the standard O(n) three-way-partition writeup. This PR adds essentially the same algorithm under a new path (data_structures/arrays/dutch_national_flag_sort.py), so it's redundant.

On the broader question — I agree it's low-value as a standalone entry. Its one genuinely instructive idea (three-way partitioning) is already better contextualized in sorts/quick_sort_3_partition.py, which shows why the partition matters. So I wouldn't add a second copy. Thanks to the author regardless for the effort.