Doc: Discuss platform plugins in deployment documentation

The requirement to deploy the QPA platform plugin as part of
the application package was not highlighted clearly enough.
This change adds a paragraph for each of the main platforms.

Also fixes linking issue with Mac OS X deployment docs and
updates information about the macdeployqt tool.

Task-number: QTBUG-34274
Change-Id: I9554bd06aa85d4f85b6f32a574efedf424117076
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>