Nested elements should show interface implemented classes

Elements that have nested elements which refer to an interface have no way of showing what classes fall under that interface.
The frankdoc json should include a list of classes that do implement such an interface.

A couple of examples of this are
- https://frankdoc.frankframework.org/#/Components/Other/Scheduler (with interface [IJob](https://javadoc.frankframework.org/org/frankframework/scheduler/job/IJob.html))
- https://frankdoc.frankframework.org/#/Components/Pipes/SenderPipe (with interfac [ISender](https://frankdoc.frankframework.org/#/Components/Pipes/SenderPipe))

Start by tracing the code that builds frankdoc JSON for nested elements, then compare the Scheduler/IJob and SenderPipe/ISender examples in the issue. Done means the JSON includes the classes implementing each referenced interface and the generated documentation can show them.

Written by the indexing model from the issue text.