Document process for using Support's promotions spreadsheet and for using a promotion doc copy

Why is this change being made?

The Promoting a Support Engineer page describes an outdated process and is unclear in certain areas as well. This MR updates the page to:

1. Refer to the People Group's Promotion Process page rather than to BambooHR
2. Explain when and how to use Support's Promotion Projections spreadsheet
3. Distinguish more clearly which work is to be done in the candidate's original (public) promotion document versus in the private copy

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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