Essential Modules: Document how to use module from CMake

We should promote CMake a first-class build system for Qt also in our documentation.

• Qt library landing pages should mention how to link against the library. That is, To link against the Qt XXX module, add this line to the project file should also mention CMake. See e.g. Qt Multimedia

• The module specific header for class documentation should also mention CMake, see e.g. http://doc-snapshots.qt.io/qt5-5.12/qaudiooutput.html[QAudioOutput Class].

Acceptance Criteria: Landing page of essential modules in https://doc-snapshots.qt.io/qt5-5.15/qtmodules.html should document how to link against the module in CMake.