Clarify PrivateLink Setup

[natalyjazzviolin]

This adds clarification for a smoother PrivateLink setup process. So far this:

• Specifies that a CLickHouse Cloud API key is needed.
• Calls out that the initial privateDnsHostname is a placeholder.
• Calls out to check that regions match in the AWS Console for service name verification.

[robot-ch-test-poll1]

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[tsolodov]

let's update GCP PSC doc using the same notes?

[tsolodov]

AFAIK we have only 2 roles(atm): developer and admin, shall we mention explicitly to create key w/ admin access?

[natalyjazzviolin]

That makes sense to me, @konsta-danyliuk , what do you think?

[konsta-danyliuk]

@natalyjazzviolin , yep, this seems more straightforward.

@tsolodov, If the Dev key type doesn't have sufficient permissions for Private Links set up, we should include the key type in the doc.