Document Geo installation on staging.gitlab.com

Why is this change being made?

This MR document how the Geo installation on staging.gitlab.com was done and what decisions are involved so that it's easy to address or troubleshoot at a later date.

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