Clarify process for triggering dev-escalation

Why is this change being made?

• The existing wording is vague about where dev escalations fit in the hierarchy of requesting help during an emergency. This Slack conversation with Support Engineers experienced with CEOC clarified that in practice, it is a last resort and isn't something the on-call engineer would/should do without the on-call manager's input.
• It isn't immediately how to trigger a dev escalation. This MR links to the section of the Handbook which explains how to trigger a dev escalation, and the steps that will follow.
• It also adds a couple links to the larger section about getting help from SE subject-matter experts and developer teams.

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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