Adding Customer Emergency Training resources

Why is this change being made?

Customer Emergency section of handbook was missing resources for training and shadowing. Adding in shadowing as way to train up. Additionally the way to add to shadow rotation has changed gitlab-com/support/support-team-meta#4442 (closed)

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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