Add caution for public comms during Incidents

Why is this change being made?

When reviewing April's Availability report, Reliability Leadership noted that there were a lot of public communications (in the form of status.io updates and Tweets) that went out, despite those incidents having minimal customer impact. This has the effect of a public perception of Reliability problems when in reality there was little to no impact to customer workflows.

This MR adds some initial guidelines for IM and CMOC to confirm customer impact with the EOC before sending out public communications.

Once this MR is approved, we need to communicate it out in the #imoc_general channel to ensure current and onboarding IMS are aware. We will also want to engage with Support to update and socialize out this understanding in their documentation as well.

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.

---