Doc: Move example docs related to QtQml under the correct module

This change moves 'Extending QML' qdoc file and the tutorials
from examples/quick to examples/qml, which is the correct
location for them.

Change-Id: I24bd9dfa75baaab7bc8efe08489f09a7b65d98a6
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>