Add part about sabbatical to the PTO handbook

Why is this change being made?

I would like to propose adding "SABBATICAL" to our paid time off policy.

Sabbaticals are offered by numerous companies: example, example, example - according to the 2018 Employee Benefits raport, it was offered by 5% of the companies (page 24).

The advantages of sabbaticals:

• Improved employee retention
• Opportunities to train new leaders (succession planning under stress)
• Increased creativity and innovation
• A more well-rounded, satisfied workforce
• Reduces cost of employee turnover Source

I think sabbatical can also be a great tool to fight burn-out, which is a huge topic in post-pandemic world (for example, it was discussed multiple times in Women TMRG).

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.

---