doc: add example showing array-based fields

doc/12-Handling-custom-variables.md

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+Working with custom variables
+=============================
+
+Icinga Director allows you to work with custom variables in a very
+powerful way. It implements the concept of `Data fields`. If you want
+your users to be able to fill specific custom variables, you need to
+add corresponding `fields` to 
+
+Examples
+--------
+* Add fields for existing commands
+* Allow to fill an [array of interfaces](14-Fields-example-interfaces-array.md)
+

doc/14-Fields-example-interfaces-array.md

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+<a id="Fields-example-interfaces-array"></a>Working with fields - interfaces array example
+==============================================
+
+This example wants to show you how to make use of the `Array` data type
+when creating fields for custom variables. First, please got to the `Dashboard`
+and choose the `Define data fields` dashlet:
+
+![Dashboard - Define data fields](screenshot/director/14_fields-for-interfaces/141_define_datafields.png)
+
+Then create a new data field and select `Array` as it's data type:
+
+![Define data field - Array](screenshot/director/14_fields-for-interfaces/142_add_datafield.png)
+
+Then create a new `Host template` (or use an existing one):
+
+![Define host template](screenshot/director/14_fields-for-interfaces/143_add_host_template.png)
+
+Now add your formerly created data field to your template:
+
+![Add field to template](screenshot/director/14_fields-for-interfaces/144_add_template_field.png)
+
+That's it, now you are ready to create your first corresponding host. Once
+you add your formerly created template, a new form field for your custom
+variable will show up:
+
+![Create host with given field](screenshot/director/14_fields-for-interfaces/145_create_host.png)
+
+Have a look at the config preview, it will show you how your `Array`-based
+custom variable will look like once deployed:
+
+![Host config preview with Array](screenshot/director/14_fields-for-interfaces/146_config_preview.png)