Add Parquet Timestamp output configuration to docs

Spark 2.3.0 added the spark.sql.parquet.outputTimestampType configuration option for controlling the underlying datatype used when writing Timestamp column types into parquet files. This option is helpful for compatibility with external systems that need to read the output from Spark.

This was never exposed in the documentation. Fix should be applied to docs for both the next 3.x release and 2.4.x release.