doc: application development: update example-application text

The application now supports a custom west extension; mention that.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

Author: mbolivar-nordic

doc/develop/application/index.rst

@@ -169,6 +169,7 @@ use of features commonly used in applications such as:
 - Custom devicetree bindings
 - Custom drivers
 - Continuous Integration (CI) setup
+- An example west :ref:`extension <west-extensions>` command
 
 Creating an Application
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