Revamp "Qt Resource System" overview

https://doc.qt.io/qt-6/resources.html is the canonical place to explain the Qt resource system. I think it would benefit from a revamp though:

• It misses a high-level overview on the features and use cases of the Qt resource system.
• It starts right with .qrc files, and explains early on explains advanced features. Nowadays it's not mandatory to even use .qrc files anymore in qmake and CMake. So the description of .qrc file and its features should should be an advanced detail, not the start.
• It focuses only on qmake, leaving out CMake API related to the resource system (AUTORCC, qt_add_resources...)
• The difference between the ":/"syntax to access resource files via QFile, and the "qrc:/" prefix for QUrl's, is confusing and should be explained.