docs: update `Gateway address unavailable` doc to include troubleshooting steps for CK8s

[sinapah]

Please describe the question or issue you're facing with "Troubleshooting Gateway address unavailable - Observability documentation".
The current doc on Troubleshooting Gateway address unavailable has instructions on troubleshooting which are Mk8s focused. These should be updated so they also include Ck8s instructions.

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Reported from: https://documentation.ubuntu.com/observability/latest/how-to/troubleshooting/troubleshoot-gateway-address-unavailable/