Add cases to Compliance verification steps

Why is this change being made?

This MR adds to the Verification section of the Compliance group page. The goal is to add context for cases where verification of MRs in staging and/or production is difficult or impossible to reproduce with normal user access. This has come up, for example, in verifying instance-level audit events that require admin permissions to generate, and would not be ideal to create in a production environment.

As discussed with @nrosandich, suggested note here is that the DRI should be responsible for determining the verifiability of an MR, and is trusted with discussing with relevant domain experts on getting correct verification steps. The verifying engineer should also sign off on cases where verification is deemed unfeasible.

@gitlab-org/govern/compliance Feedback is appreciated.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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