Make 'content' target name unique when generating a QDS project

Recently we had some design studio generated projects added as examples to the qt/qtdoc repo.

The problem is that the generated projects create a target called 'content', which is the same across any project. CMake expects unique target names, so as soon as a 2nd 'content' target is created, it fails configuration of qt5.git CI.

See https://bugreports.qt.io/browse/QTBUG-115343

Here's an example project:

https://github.com/qt/qtdoc/blob/dev/examples/demos/FX_Material_Showroom/content/CMakeLists.txt

It would be good if the project generator created a unique target name, for example FX_Material_Showroom_content'.
This will avoid future errors when a QDS project is added as an example to qt.