Add Enterprise user domain verification instructions

What does this MR do?

Add instructions on how to view, add, and manage configured domains in the group.

A customer was very upset thinking they had to configure the A record to get verified domain. However, that's not necessary. I decided to add instructions on how to view and manage domains since those were missing.

The instructions will change once we implement #375492 (closed) but all the more so we'll need a specific section of the docs once that feature is done.

Related issues

Related to https://gitlab.zendesk.com/agent/tickets/379838 (internal)

Screenshot

For reference, this is what a domain verification page looks like. It's only available at the top-level group. This one shows how a verified and an unverified domain looks like:

Author's checklist

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• [-] If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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