Update Elevate handbook page with structure, FAQ, and manager guidance

Why is this change being made?

We're getting ready to launch new cohorts of Elevate. This MR will update the handbook page to provide resources for participants when the new cohorts open. The changes include:

1. Updates to structure of program to reflect participant responsibilities
2. Update language of 'pilot' and reiterate FY24 goal to graduate 85% of managers
3. Include new FAQ section
4. Include new support section for leaders who have direct reports participating in Elevate

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, and the content is SAFE
• 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
  • The when to get approval handbook section explains the workflow in more detail
• 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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