Add an example 'Table of Contents' to show how to create and use custom
tree model (derived from QAbstractItemModel) in the Quick TreeView
control.
Fixes: QTBUG-127016
Pick-to: 6.8
Change-Id: I9a5d9e837e1b18de322a0ec895d9832918fd5816
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>