Missing manual steps in PrivateLink config

[WanYixian]

Related code PR

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Which part(s) of the docs might be affected or should be updated? And how?

Currently, our PrivateLink feature doesn’t work out of box for Confluent private Kafka clusters. Two extra steps needs to be performed (manually):

1. Before provisioning a RW PrivateLink, we need to make sure the Availability Zones of their cluster is a subset of the Availability Zones RisingWave offers for PrivateLink provisioning
2. We need to manually add DNS records for the PrivateLink after the it’s provisioned.
   Before we can automate these steps, add some explanations to the existing PrivateLink doc and ask user to contact us first if they plan to use the PrivateLink to connect to a Confluent Cloud cluster.

Reference

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