Add missing Prometheus collectd example in our documenation

The current docs don't provide a collectd-configuration example.

Also this important entry is not in the default configuration that is provided by an OpenNMS installation.

We should add an example like this: 

 
<service name="Prometheus-Node-Exporter" interval="300000" user-defined="false" status="on"> 
 <parameter key="collection" value="node_exporter" />
 <parameter key="thresholding-enabled" value="true" />
 <parameter key="url" value="http://{ipaddr}:9100/metrics" />
 </service>
<collector service="Prometheus-Node-Exporter" class-name="org.opennms.netmgt.collectd.prometheus.PrometheusCollector"/>   

I'm not sure right now how this service should be called officially. Do we have an official name?
Could also be `Node-Exporter` or when talking about other exporters `Jitsi-Exporter`, `Ceph-Exporter` and so on.