doc: Add severity level explanations

Author: sebix

intelmq/etc/harmonization.conf

@@ -254,7 +254,7 @@
             "type": "URL"
         },
         "severity": {
-            "description": "Severity of the event, based on the information from the source, and eventually modified by IntelMQ during processing. Meaning of the levels may differ based on the event source.",
+            "description": "Severity of the event, based on the information from the source, and eventually modified by IntelMQ during processing. Meaning of the levels may differ based on the event source. Allowed values: critical (highly critical vulnerabilities being actively exploited and pose a very high likelihood of compromise. For example, RCEs, sensitive data access), high (end of life systems, accessible internal systems that should not be exposed, risk of data leaks, malware drone and sinkhole events), medium (DDoS-amplifiers, unencrypted services requiring login, vulnerabilities requiring MITM to exploit, attacks need prior knowledge), low (deviation from best practice, little to no practical way to exploit, but setup is not ideal), info (informational only, no known risk), undefined (unknown or undetermined)",
             "length": 10,
             "regex": "^(critical|high|medium|low|info|undefined)$",
             "type": "LowercaseString"