Adding promotion document template

Why is this change being made?

Adding a promotion document template to the handbook - Largely inspired by @dsatcher (thank you!)

Per our discussion during our PBP weekly meeting, there has been feedback that promotion documents can be inconsistent, which can cause a sense of "unfairness" and inconsistency in the promotion process.

As a first iteration, I am proposing to add a general promotion document template that can serve as a baseline for what all promotion documents should include.

For future iterations, we can consider including department/division/role specific promotion document templates and examples. @dsatcher has an initiative started for this for Development (specifically Create) here: #8104 (closed)

@gl-peoplepartners would love your feedback on:

• The general promotion template
• Our guideline for the length of promotion documents (~5-7 pages?)

Author Checklist

• Correct MR template applied (e.g blog post)
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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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