Update portal handbook page with subheader

Why is this change being made?

Update portal handbook page with subheader

https://about.gitlab.com/handbook/business-ops/business_systems/portal/#adding-new-skus

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