SEO-enhance attached property documentation

Currently, it's rather hard to find the documentation on how to write
attached properties. The page that gets found first in most search
engines seems to be the example - so link from it to the actual
documentation pages.

Pick-to: 6.2 6.4
Change-Id: I78c9949d62863b5c11ffcd97413084ab03b0bc33
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

examples/qml/doc/src/qml-extending.qdoc

@@ -240,6 +240,12 @@ This example builds on:
 \brief Attached Properties.
 \ingroup qmlextendingexamples
 
+This example demonstrates how to create custom
+\l {Attached Properties and Attached Signal Handlers} {attached properties}.
+For a more in-depth description on how one can create attached properties,
+see \l {Providing Attached Properties}.
+
+
 This example builds on:
 \list
 \li \l {Extending QML - Grouped Properties Example}