Display forms in a modal dialog with Drupal 8

Drupal 8 has a great AJAX form API which includes some tools to create modal dialogs using the jQuery modal library. The Examples module even demonstrates how to create a custom form and display it in a modal window. But what if what you want to do is display an already created form in a modal? How do we do that? Let's see how to do it with an example. Let's display the node add form in a modal window.