Document internal Qt CMake API

We should have documentation about the key concepts and internal functions that we use to build Qt.

Things like qt_internal_add_module, qt_internal_add_resource, qt_build_repo.
Mention how qt_internal_add_resource differs from qt_add_resource.
Mention that static qt plugin target names need to end with a Plugin suffix.
Mention qt_feature, all the various options that it can take like CONDITION, AUTODETECT, ENABLE, and also how passing configure args like FEATURE_foo and INPUT_foo affect feature selection.

And many more other things.