Clarify Issue Triage DRIs for Adding Severity

Why is this change being made?

There has been confusion lately around who is responsible for adding severity labels to issues during triage. This change adds documentation to clarify that the Contributor Success team is currently responsible for partial triage, which includes adding a severity label.

As suggested by @vincywilson in this internal Slack thread, if the Contributor Success team needs additional context in order to accurately add severity, then the SET for that product group can help assist with triage (or QEM, if no SET is assigned).

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.
• 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
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• 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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