Add new Geo guide to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!118103 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://pw-add-geo-setup-to-nav.docs.gitlab-review.app/ee/administration/geo/setup/two_single_node_sites.html