Creating Orders for Add-ons to multi-year Subscriptions

Why is this change being made?

This is to add clarity on how to build orders with annual payments for multi-year subscriptions as per https://gitlab.com/gitlab-com/finance/issues/1575

@mgilgalindo @jrabbits @atkach1 This was discussed in our bi-weekly billing/DD sync. Please let me know if this is ok or would you like anything else added or updated in relation to this? Thank you

@msnow1 I was told that you can merge into the Order Processing page so assigning to you so this can be added to the handbook. Thanks

cc @csotomango

Does this MR meet the acceptance criteria?

Conformity

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