Impala 2.13 Doc: ORC file format support

ORC is a columnar file format much like Parquet. Docs: https://orc.apache.org/docs/index.html

Currently, we only support reading primitive types in ORC files (on-going works are tracked at IMPALA-6943). It's still an experimental feature which can be disabled by setting startup option --enable_orc_scanner to false.