Properly document license of Qt examples

So far we only referenced
http://opensource.org/licenses/bsd-license.php

,

which is actually not the BSD-3-clause license we use,
but BSD-2-clause.

While at it, also document the license itself as part
of the documentation, and reference it from the
licensing entry page.

Change-Id: I877dde5b8a1f1c2dfbdc01665a2a9965a997552d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>