Add Info on How to Verify Account Ownership to account_verification_html.md

Why is this change being made?

While processing data deletion requests in service desk, I couldn't easily find the steps required to evaluate account verification information. I talked with @bcarranza who informed me that those with admin access can use https://gitlab.com/admin/users/USERNAME, and after talking with @cynthia I was reminded that one can also use chatops to verify users, in addition to the ZD GitLab User Lookup App (used for 2FA resets and other Zendesk tickets that require ownership verification).

This MR adds a single line to the Evaluating Challenge Answers section of account_verification_html.md briefly describing these methods as well as the method used in Zendesk (Zendesk GitLab User Lookup App).

The intention here is to have that basic information available in the same place as the rest of the workflow that support engineers use for account verification, to simplify the amount of information gathering and searching required for these issues. This will especially help out newer support engineers.

Author Checklist

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