Ensure that Qt6 module documentation is complete

As a developer using Qt 6, I would like the documentation set to include relevant info. about the modules and the examples that are shipped with Qt 6.

Considering that Qt 6 will be a smaller subset of Qt5, the following changes are required in the existing Qt 5 documentation:

• Modules list in the All modules page is up-to-date.

For more information about the list modules that are proposed to be part of Qt6, see https://intranet.qt.io/display/QTRD/Discussion+of+Qt+6.0%2C+6.1%2C+6.2+Supported+Functionality

NOTE: Verify the proposed modules list are reflected in the .gitmodules file in qt5.git (6.0 branch).