Resolve "Member based MR rate vs development based MR rate"

Why is this change being made?

Add new dev KPI metric to handbook: Member-based MR rate vs Development MR rate.

How will this merge request impact page speed?

It does not.

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

Closes #7177 (closed)