Update Quoting Channel Deals section ahead of disti quoting transition pre announcement

Why is this change being made?

This change is to update the Quoting Channel Deals section of the Deal Desk handbook prior to the disti quoting transition pre announcement. These sections are referenced in the announcement/enablement materials, so need to ensure they're updated ahead of the announcement. Note:

• There's nothing Quote Studio specific in these sections (i.e., shouldn't be a problem to merge now, prior to QS launch)
• These changes were included in the DD mega-merge that will go live on Apr 3, but need to pull forward to align to pre-announce timing

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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