Doc: Set example install path in documentation config

Correct example install path is required for the example
manifest file to contain valid paths to the .pro files.

Task-number: QTBUG-38359
Change-Id: Ibfe98ed4f3402f2e1534b578d0caecf3676a6f36
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>