Add specifics around Engineering Managers to product dev workflow

Why is this change being made?

The Product Development flow uses "Engineering Manager" in a broad sense. This MR attempts to make the language more specific where necessary, leaving the broad term "Engineering Manager" to reference all managers within Engineering.

This MR also updates instances of "Engineering team" to use "Development team".

Product Design Manager additions

• Added to "Who" under Validation goals & outcomes
• Added to "Collaborator" under Validation backlog
• Added to "Informed" under Solution Validation
• Added to "Collaborator" under Plan

Author Checklist

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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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