Add link to Qt DC documentation to Supported Platforms page

Supported Platforms page (http://doc.qt.io/qt-5/supported-platforms.html

)
needs to contains a link to the Supported Target Devices page in
Qt for Device Creation documentation.

Task-number: QTBUG-61828
Change-Id: I1f42d3efd7359fe8da81241c9b1e4ae59240500b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

doc/src/platforms/supported-platforms.qdoc

@@ -65,6 +65,9 @@
     In the above table, the configurations in \b bold are \e {reference
     configurations}.
 
+    \b {See also}: \l {http://doc.qt.io/QtForDeviceCreation/qtee-supported-platforms.html}
+                      {Qt for Device Creation: Supported Target Devices and Development Hosts}.
+
     \section2 Reference Configurations
 
     The reference configurations are run on the continuous integration (CI)