[Docs] Tutorial on making a custom detect trends function in BigQuery Python UDF, and Databricks

[RussellSB]

Saw online that with python udfs you can make SQL functions in big query that pass and return data to/from Python.

This could be game changing for fellow big query users... will hop on the recent trend of forecasting, outlier detection, etc sql functions getting recently made available.

I'm not sure if this is definetely possible in Databricks too. But potentially! Worth investigating once this can get picked up.