Document "RawControl"

Some edge case could be nicely handled by renderer which are not bound to a JSON Schema. This can be implemented in a rather straightforward manner but is somewhat out of scope to be an out-of-the-box renderer as any actual use case will be slightly different, leading us to reimplement the whole renderer set without schema support.

However we should document the approach so any user can implement one on their own.

Example use cases:
* https://spectrum.chat/jsonforms/general/render-an-array-of-radio-elements-with-other-option~f3bf3225-4ed8-496d-9508-c929e40baf0e
* https://spectrum.chat/jsonforms/general/render-a-list-and-be-able-to-select-an-item-from-that-list~bc55b950-d4fd-4048-b2f6-3a460a589be4