migrate: convert joins.ipynb from pandas to polars

[meshackamadi]

Summary

Converts the joins.ipynb chapter from pandas to polars, covering DataFrame concatenation and database-style joins.

Changes

Code

• pd.concat() → pl.concat()
• pd.merge() → .join() method
• Added semi and anti join examples
• Stata files: read with pandas, convert with pl.from_pandas()
• Manual indicator column using pl.when()/pl.then()

Text

• Updated prerequisites to mention polars
• Changed "merge" to "join" throughout
• Updated documentation links to polars docs

Testing

• All code blocks execute successfully
• All join types produce expected results
• Notebook renders without errors

Files Modified

• joins.ipynb