ENH: Review and update `prep` folder

[pavithraes]

The prep folder contains some rough/draft scripts and notebooks we used to prepare the tutorial materials, these need to be reviewed and updated with descriptive text to be useful as reference material for learners:

• airline_data.py
• csv_to_parquet.ipynb
• csv_to_parquet_sorted.ipynb
• store_dtypes.ipynb

This folder also has a couple of "data" files, that we can consider moving into a separate "data" folder:

• dtypes.json
• airports.csv