Define INT96 ordering

Currently int96 binary ordering doesn't match its natural ordering.
We should either specify this or declare int96 not ordered and link to the type replacing it.

**Reporter**: [Julien Le Dem](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=julienledem) / @julienledem
#### Related issues:
- [INT96 should be marked as deprecated](https://github.com/apache/parquet-format/issues/301) (is related to)

_{**Note**: *This issue was originally created as [PARQUET-904](https://issues.apache.org/jira/browse/PARQUET-904). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by reading the related INT96 deprecation issue (#301) and the linked migration documentation for context on its replacement. Determine whether the format should define natural binary ordering or explicitly declare INT96 unordered, then document the chosen behavior and link the replacement type.

Written by the indexing model from the issue text.