migrate: convert numbers.ipynb from pandas to polars

[meshackamadi]

Summary

Converts the numbers.ipynb chapter from pandas to polars, covering numeric operations, transformations, and summary statistics.

Changes

Code

• pd.read_csv() → pl.read_csv()
• df.groupby() → df.group_by()
• .assign() → .with_columns()
• .query() → .filter()
• .apply() → .map_elements()
• Row operations: axis=1 → pl.min_horizontal()
• .cumsum() → .cum_sum()
• .isnull() → .is_null()

Text

• Updated prerequisites to mention polars
• Updated documentation references
• Changed function names to polars equivalents

Testing

• All code blocks execute successfully
• Numeric transformations produce expected results
• Notebook renders without errors

Files Modified

• numbers.ipynb