[doc] Add ADR for dynamic widgets support in Forms

[pcdavid]

Known issues which needs to be addressed:

• Now that a single WidgetDescription can produce 0, 1 or many instances, the widgets' id, as provided by the WidgetIdCounter, can not be relied upon. Between two successive renders of the same Form, the widget with id #4 can be completely different ones (different instances of the same widget description, or instance of completely different widgets; maybe of the same kind maybe not).
• FormDescriptionEditor support: rendering is easy, but the result does not actually support all the FDE operations (add/remove/move widgets). A FormDescriptionEditor normally display a plain Form instance, with the actual widgets rendered on sample/preview data, but DynamicFor & DynamicIf, while they exist on the FormDescription, do not exist as widgets in the actual rendered Form. I guess it would be possible to create dedicated widgets for them (rendered how? as containers?) and only render them when some flag/variable is present, but that makes the core Form dialect know about a "special mode" just for the FDE).