Document add-on process for subscription orders through GCP private offer

Why is this change being made?

We currently do not have documentation clarifying the process when a customer needs to upgrade or add users to a subscription originally created through a Private Offer on the Google Cloud Marketplace.

Recent confirmation with Deal Desk and Alliances states this requires a contract reset every time of which we should add to the handbook overview.

This will make it clear on expectations for working through the Private Offer process how to do additional orders if needed.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)