samples: application_development: add link to example-application

Because example-application contains relevant sample code related to
application development.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

samples/application_development/application_development.rst

@@ -3,8 +3,14 @@
 Application development samples
 ###############################
 
+For application development you should also consider looking at
+`example-application`_. Among others, you will find on it out of tree driver
+or board examples.
+
 .. toctree::
    :maxdepth: 1
    :glob:
 
    **/*
+
+.. _example-application: https://github.com/zephyrproject-rtos/example-application