Draft: Add guidance for severity of SUS-impacting issues that add new features

Why is this change being made?

As discussed in the UX Weekly Meeting Jan 18 (internal), adding severity labels to SUS-impacting issues that describe new capabilities that do not fall under the existing guidance can be confusing.

This MR introduces guidance to add severity labels to these issues, and how to do so. Thanks @beckalippert for raising this point!

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