Add need for event expense tag to TMRG handbook

Why is this change being made?

This is the process I've followed in the past when using Expensify for TMRG events, so adding it to the TMRG handbook.

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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  • The when to get approval handbook section explains the workflow in more detail
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  • 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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