Add documentation for building a custom container-based widget

Prerequisites • I have searched for similar feature requests in both open and closed discussions and cannot find an existing request. • I have verified that the feature is still missing from the latest stable versions of Elementor or Elementor Pro. What problem is your feature request going to solve? Please describe.. There's not a good explanation for developing container-based widgets. We can't always wait for Elementor to release the features we need or fix the bugs we've found in container-based widgets. Some features are too custom and will never be included. This is why we need this documentation. Describe the solution you'd like. A simple explanation for which classes to inherit and/or methods to override in order to build a widget that supports nested containers. I specifically need a guide on creating a loop widget with loop item template support. Describe alternatives you've considered. Other 3rd-party widget plugins may have the f…