docs(openshift): add manual JWT signing secret step

[natifridman]

Summary

• Adds a missing step to the OpenShift install guide: manually creating the openshell-jwt-keys secret before installing the chart.
• When pkiInitJob.enabled=false, the PKI init job that normally generates this secret is skipped, leaving the gateway pod stuck in ContainerCreating with MountVolume.SetUp failed for volume "sandbox-jwt" : secret "openshell-jwt-keys" not found.

Test plan

• Followed the updated steps on an OpenShift 4.x cluster — gateway pod starts successfully after creating the secret.

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