Add section about customers portal staging access

Why is this change being made?

To document how team members can gain access to customers portal staging when they need to test functionality.

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