add guide to enable central logging to elastic

admin-guide/configuration/central_logging_elastic.md

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+# Configure Central Logging to Elastic
+
+To ship the logs of a system to the central logging service (Elastic) following pre steps are needed:
+
+1. Have a space in Elastic to ship the logs to
+2. Have a space API key
+
+Both prerequisites you can check with / talk to Michel Rebmann (michel.rebmann@psi.ch) / Group 9522
+
+
+Afterwards the log shipping can be configures as follows in hiera:
+
+```
+base::enable_elastic: true
+elastic::space: 'lx'
+elastic::space_api_key: 'yoursuperstrangeapikey'
+```
+
+```{note}
+Replace the space name as well as the space_api_key according to your setup.
+```
+
+By default __all__ journald logs are shipped to the central Elastic instance. If you want to __limit__ the logs to specific units, the units can be specified as follows:
+
+```
+elastic::systemd_units:
+    - 'sshd.service'
+```