Document the 4 new panels

[christian-konrad]

What should we do?

Document the 4 new panels, especially the mock data input panel.

Probably create a new page and call it "Validate your forms".

It could be place here: https://docs.camunda.io/docs/components/modeler/forms/camunda-forms-reference/

And referenced here: https://docs.camunda.io/docs/guides/utilizing-forms/

Why should we do it?

To describe the scope of potential benefits of the new preview and mock data panels, and to describe the limits (e.g., so people won't expect that the data in the input panel is persisted in the form).