Add EM manual verification instructions to release post template

Why is this change being made?

This adds some additional guidance for Engineering Managers to the Merge Request template for Release Post Items. Specifically, it adds a checklist item for manual verification and issue closure. As a bonus it adds a link to a helpful documentation page.

This was prompted by a suggestion from @stkerr.

Edit for clarity: In a 1:1 call, @stkerr observed that we probably don't want to announce features that have not been verified, and @djensen agreed.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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