Refactor the "Extending the Interfaceframework" page

The documentation contains a page called "Extending the ..." (https://doc.qt.io/QtInterfaceFramework/extending-interfaceframework.html).

Other than the title implies, the page isn't really about how to extend the module, but a mixture of how it works and what you shouldn't do.

This page should be refactored and if things make sense to keep, they should be moved into other parts of the documentation instead.