Add instructions for setting up Duo Pro on staging Gitlab.com

What does this MR do and why?

This MR adds instructions for setting up Duo Pro on a staging.gitlab.com account.

To be used in cases like this: https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/30472#note_2012747336

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A