Document existing support for "hierarchical" output

[handrews]

There are two new output formats proposed for the next version of JSON Schema, with new names so they can be added alongside the existing ones, which will be deprecated (except for "flag"). There is no technical reason not to support them for 2019-09 and 2020-12 as well.

This issue tracks the "hierarchical" format, which has a few details yet to be resolved per json-schema-org/json-schema-spec#1319

[handrews]

Looks like hierarchical is already supported, but is not listed in the documentation. So I'll just convert this issue to be about documenting it (@marksparkza unless there's a reason it's not included in the docs for the output() method?)