Update docs after modularization of docs

Update docs after modularization of docs

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

We now use QMAKE_DOCS to trigger doc generation instead of rolling our
own rules for qdoc.

Change-Id: I900029d71c305a1f269cddeeb0e05c524d933834
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>