✨ Introduce quick start + diagram for GitLab Dedicated

Why is this change being made?

This MR introduces a Quick Start workflow for Support Engineers. While this does not replace the GitLab Dedicated support training module, it acts as a supplement.

This is intended to be a quick reference for Support Engineers who are hesitant about taking assignment of GitLab Dedicated tickets. The Quick Start guide should let them know what they need in order to take assignment or send a next response.

See gitlab-com/gl-infra/gitlab-dedicated/team#2319 for the conversation and the origin of the diagram.

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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