Update troubleshooting guide to include remediation for incorrect pre…

[BenjaminBraunDev]

Due to customer issues with TTFT spikes caused by the prefix cache scorer having an incorrect configuration, adding this to the troubleshooting guide to make it easier for users to diagnose and remediate similar issues.

In this case it was unclear that the TTFT spikes were caused by the prefix cache config until we saw the config wasn't set to the right parameters for the model being served.

Does this PR introduce a user-facing change?:

NONE

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[liu-cong]

Since v1.2 the plugin auto tunes such configurations from the model server metrics so no manual tuning is required, #1748. We should recommend users using the v1.2+ versions, and highlight that such tuning is only required before v1.2