Add guidance on `severity::2` Incidents

Why is this change being made?

In reviewing past SLA drops I've noticed a few incidents marked as severity3 or severity4 even though the overall availability metric was negatively impacted. As a result, post incident actions were not properly prioritized. This MR updates the incident management page to indicate that any incident that has a negative impact on the Overall SLA should be at least a "severity::2" incident.

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)

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