Improve Geo Disaster Recovery documentation for systems with multiple secondaries

When we've been running through the Geo DR demo, the command to promote the secondary node to primary display a warning message that mentions for systems with multiple secondaries they should follow the documentation in https://docs.gitlab.com/ee/gitlab-geo/disaster-recovery.html, but there’s no actual documentation just a small warning inside the first step.

Proposal

Add a new section that makes clear that we don't support systems with multiple secondary geo replicas yet and display the correct link on the command to promote the secondary node to a primary or improve the warming message and remove the link to the docs.