Convert categorical operations from pandas to Polars

[meshackamadi]

Summary

This PR migrates categorical data examples from pandas to Polars and updates the accompanying explanations to better reflect Polars' API and behavior.

Changes

• Replaced pandas categorical operations with Polars equivalents.
• Updated examples to use pl.Enum for ordered categorical data.
• Improved explanations of category operations in Polars.
• Added examples demonstrating:
  • value_counts()
  • retrieving Enum categories with .dtype.categories
  • including zero-count categories using a join
  • mode() on Enum columns
• Updated examples to follow current Polars idioms.
• Corrected documentation to reflect Polars' stricter handling of Enum categories.

Notes

• value_counts() only returns categories that are present in the data.
• Added an example showing how to include unused categories by joining the Enum category list with observed counts.
• Removed pandas-specific categorical behavior that does not have a direct Polars equivalent.