The stream-based XML serialization API resides in corelib/serialization.
Move the steambookmarks example there. The Qt XML documentation is
updated to no longer refer to this example code directly and refer to
the direct location in the example documentation instead.
Task-number: QTBUG-110647
Pick-to: 6.5
Change-Id: Id36fb04a6acb7b8d1eb008f61568fe0abc221e3d
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
