[Docs][Java] Dataset Javadocs are not being published

The Javadocs for 7.0.0 don't list org.apache.arrow.dataset: https://arrow.apache.org/docs/java/reference/index.html

**Reporter**: [David Li](https://issues.apache.org/jira/browse/ARROW-15702) / @lidavidm

_{**Note**: *This issue was originally created as [ARROW-15702](https://issues.apache.org/jira/browse/ARROW-15702). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start at the published 7.0.0 Java reference page and trace how the Java Javadocs are generated and published. Check why org.apache.arrow.dataset is absent, then verify that the published reference index lists it after the documentation process runs.

Written by the indexing model from the issue text.