Refactor parquet datasource state machine

Author: alamb

datafusion/datasource-parquet/src/mod.rs

@@ -15,6 +15,10 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! DataFusion Parquet Reader: [`ParquetSource`]
+//!
+//! [`ParquetSource`]: source::ParquetSource
+
 // Make sure fast / cheap clones on Arc are explicit:
 // https://github.com/apache/datafusion/issues/11143
 #![cfg_attr(not(test), deny(clippy::clone_on_ref_ptr))]