Doc: Define example install path in .qdocconf

Example install path under QT_INSTALL_EXAMPLES needs to
be defined in .qdocconf to generate valid projectPath
values in example manifest files.

Change-Id: I870cde7f416f465f06e6534c94e92b02b32e1b41
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>