Support Time Off - update F&F Days, ensure notice period, consult SGG calendars

Why is this change being made?

The current Support Time Off page lacks concrete guidance on how Support observes Family & Friends Days. In general, engineers are expected not to take the "official" day, but to spread out our alternate days to ensure adequate staffing (since our customers are not off work). This MR adds specific guidance for F&F Days.

This MR also emphasizes the company policy about 2x notice period (ex.: 6 days notice for 3 days off), and provides steps on how to consult your SGG's upcoming OOOs to assess the impact of your planned time off on your SGG.

This MR makes a handbook change with global team impact. Following the Change Management process, it is being put out for general review for 1 week. When feedback is processed, global communications will be circulated according to the Rollout Issue Template.

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • 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 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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