Doc: point the automatic links to Qt 5 docs

The index files are now fetched from qt5/qtbase/doc.
Removed the qt.index file that came from Q 4.
When building the docs with Qt 4, no links will
be generated automatically.

Change-Id: I0786745f218e4c854e08012664629a7697dcadf3
Reviewed-by: Eike Ziller <eike.ziller@digia.com>