Include manual process for dev oncall coordinator

Why is this change being made?

It's quite common for team members to be rostered on-call when they have planned PTO. This addition to the issue template for coordinators links to the manual 'how to' process, and mentions the need to check the team members calendar when scheduling. This will reduce the need for rescheduling.

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
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  • 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
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in linking to this MR
  • Mentioned in #eng-managers

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