Overhaul "QtQuick -Scalability" page

https://doc.qt.io/qt-6/scalability.html covers an important topic, the creation of applications with responsive interfaces, covering different DPI values.

Unfortunately, the current documentation has not kept up with changes/improvements in the framework. A couple of points that should be changed/investigated:

• High DPI section might need an update, at least it shouldn't only talk about OpenGL without mentioning RHI or the other native APIs
• The sections about Layouts and Orientation sound like they could benefit from some cross linking to https://doc.qt.io/qt-6/qtquicklayouts-responsive.html ,and  maybe some overhaul
• Don't we support highres images (@2x) on all platforms nowadays?
• We shouldn't promote the usage of QQmlPropertyMap; if we don't have a better pattern for exposing the data, we should rather recommend a singleton based approach

We also should double check whether the current structure of the page is ideal, and whether there can be more cross-linking in general (e.g. to the same topic for Widgets)