Chore - Use the new GCS auth helpers where possible

[raebot]

Replace calls to read_parquet, read_file, and geo_data_frame.to_parquet with GCS helpers introduced in calitp-data-analysis v2025.6.24 so it's easier to access these resources by virtue of not having to fetch a token each time.

Detailed description:

Stakeholders & End-Users

Analysts' lives will be made easier by replacing inline GCS auth token or filesystem calls with these helper functions because there will be a clear and consistent way of accessing these geospatial resources.

[raebot]

@KatrinaMKaiser do you think we should prioritize this one?

[KatrinaMKaiser]

Yes I think so, it's a fundamental and wide ranging improvement

[raebot]

• Create entry in Useful Python Libraries
• Update use in a few places for demonstration
• Confirm deployment to the docs site