Doc: Additional info needed for rpm/deb config

Topic: https://www.elastic.co/guide/en/logstash/master/running-logstash.html#running-logstash-systemd

Add more info on how rpm/deb packages work: `systemctl start logstash` won't work without additional configuration.

To Do: Add sample config for most basic use case.

cc:/ @dliappis @flexitrev @jsvd @andsel

Start with the linked running Logstash systemd documentation topic and review how rpm/deb packages are configured before running `systemctl start logstash`. Add a basic-use-case configuration example explaining the required package setup, then verify that the documentation accurately reflects the startup flow.

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