Document return from PTO strategies

Why is this change being made?

During our Live Learning with John Fitch, we discussed how GitLab team members can integrate back into their teams after taking time off. John made a comment that he thought our documentation about when/how to take time off was great, and that we could perhaps increase our documentation around strategies for team members to successfully transition back into their role and apply the reflective benefits that PTO allows. In response to that discussion, this MR

• begins a new section on the PTO page for returning from PTO
• moves 2 strategies from other locations on the page into this section
• adds a strategy based on conversations in the Live Learning
• adjusts a small edit on the page concerning a misplaced image

Thanks! FYI @Josh_Zimmerman @dmurph

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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