Restore .index files in the documentation

Task-number: QTBUG-38687
Change-Id: Ibf8ec85d3ffcc0bbdfada32469b89f9bd7e26d24
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>