Add troubleshooting section for Kubernetes Agent unknown authority error

What does this MR do?

Documents how to add a trusted internal CA certificate to the agent so that it is able to connect back to a GitLab instance using cert signed by internal CA. Relates to internal ticket query from customer who uses an internal CA.

Provides two options for incorporating CA certificate into agent config by mounting configmap as file in agent - one taking advantage of the default location for CA certs, and the other making use of the --ca-cert-file parameter.

Related issues

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