For classes, the tree of subclasses is rendered, recursively.
For interfaces, the descendants are separated into child interfaces, which
extend the interface, and classes, which implement the interface.
Closes#19306
This is to tidy up the `author-packagse`, which currently duplicates a
lot of the configuration in the main packages. We need to
DRY this up so that we don't fall foul of a change in one being missed in
the other.
