Add exit code documentation

docs/getting-started/exit-codes.md

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+---
+sidebar_position: 14
+description: Exit Codes
+---
+
+# Exit Codes
+CATS has the following exit codes:
+- `191` when for invalid input
+- `192` when something unexpected happens during execution
+- `number_of_errors` number of errors found while fuzzing
+
+:::info
+There might be cases when `191`, `192` might overlap with the actual number of errors reported by CATS.
+These are probably very rare cases. You can use the `cat cats-report/cats-summary-report.json | jq .errors` to differentiate if needed.
+:::