[GitLab Dedicated] Add info on accessing Switchboard for Support

Why is this change being made?

💡 Provide a detailed answer to the question on why this change is being proposed, in accordance with our value of Transparency.

This updates the GitLab Dedicated Switchboard Troubleshooting workflow page to includes notes on how folks in GitLab Support can access Switchboard. Now that Switchboard is GA, the GitLab Support team needs to be prepared to support customer inquiries. Ensuring that folks know how toa ccess Switchboard is an important part of this prep, especially since it's a brand new application.

See gitlab-com/gl-infra/gitlab-dedicated/team#2348 for the initial conversation about who gets access.

See gitlab-com/team-member-epics/access-requests!1465 for the request to add this access as a baseline entitlement.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• 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
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

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