Add priority and severity single source of truth to the handbook.

Why is this change being made?

We need to be handbook first and our process needs have a single source of truth to the handbook.

This MR moves all the information around Priority and Severity to the handbook.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what