Documentation : Review documentation about GeoNode URLs

Follow up for https://gitlab.com/gitlab-org/gitlab-ee/issues/9142 and https://gitlab.com/gitlab-org/gitlab-ee/issues/9174.

• https://gitlab.com/gitlab-org/gitlab-ee/blob/9d3b1195/doc/user/admin_area/geo_nodes.md.
• https://gitlab.com/gitlab-org/gitlab-ee/blob/9d3b1195/doc/api/geo_nodes.md

Make sure documentation makes sense and is helpful to customers.

• Alternate URL
• Internal URL
• URL