[Python][Parquet] Add examples of using HDFS filesystem and Parquet files together

It seems this is not clear enough in the documentation

https://stackoverflow.com/questions/57500284/how-to-write-on-hdfs-using-pyarrow

**Reporter**: [Wes McKinney](https://issues.apache.org/jira/browse/ARROW-6239) / @wesm

_{**Note**: *This issue was originally created as [ARROW-6239](https://issues.apache.org/jira/browse/ARROW-6239). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}