Add PTO guidance to Verification section

Why is this change being made?

We should include guidance on handing off active MRs when the author will be on PTO. This is captured as a follow-up in our FCL at gitlab-org/gitlab#364042 (closed)

Review app at https://nmccorrison-master-patch-16761.about.gitlab-review.app/handbook/engineering/development/sec/protect/planning/index.html#planning-for-pto

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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• 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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