Add pandas+matplotlib example (examples/library_examples/pandas_matplotlib)

[studentpiyush]

What does this PR do?

Adds a beginner-friendly pandas + matplotlib example demonstrating a small data-analysis workflow with a moving average line chart.

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change? (example code + README)

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 (or one example folder)
• All new Python files are placed inside an existing directory (examples/library_examples/ is acceptable)
• 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 (not required for examples)
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation (not needed for this example)
• 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 Title: Add Example Code Using Popular Python Libraries #13445"

How to run

pip install pandas matplotlib
python examples/library_examples/pandas_matplotlib/example.py

[cclauss]

No functions, no tests, no PEP723 header, we don't do examples, we do algorithms. Read CONTRIBUTING.md