The global files now live in qtbase only, and are made available to the
other modules via the $QT_INSTALL_DOCS variable.

There's potential missing docs in doc/qt5.qdocconf that we may need to
move into each module, or to global docs.

Change-Id: I00a5b6999359f75d0ee7514897762662ed76af18
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>