## What changes were proposed in this pull request?

Copy the package documentation from Scala/Java to Python for ML package and remove beta tags. Not super sure if we want to keep the BETA tag but since we are making it the default it seems like probably the time to remove it (happy to put it back in if we want to keep it BETA).

## How was this patch tested?

Python documentation built locally as HTML and text and verified output.

Author: Holden Karau <holden@us.ibm.com>

Closes #12883 from holdenk/SPARK-15106-add-pyspark-package-doc-for-ml.