Improve Imagine style documentation

We have a section in the Imagine style documentation that explains how to create 9-patch images, but the feedback has been that this could be improved.

• Despite not being mentioned by the Qt Imagine Style documentation page, Affinity Design has an extensive image export functionality as part of its “Export Profile” https://codereview.qt-project.org/c/qt/qtdeclarative/+/424958
• What is the expected way to generate webp animations? How were the provided ones done? With what tools? Were they converted from some other format? I couldn't find a way to do it that didn't involve creating an animation in some other format, and converting it?
• Link to the Sketch template for the default Imagine style assets in the Asset Examples section. https://codereview.qt-project.org/c/qt/qtdeclarative/+/424965
• Document the process of drawing lines for more tricky controls. E.g. Switch handle was tricky to do 9-Patch lines for; they are round and have drop shadows. Diana can provide a list of controls that were more difficult to do.