WIP: Docs/update geo docs

What does this MR do?

• Update IP address to be more clear which one is the primary and the secondary
• Add step to add external_url in gitlab.rb on the secondary node
• Add example code block to include all configurations for the primary
• Add note that a dash (-) cannot be used in the replication slot name
• Add sudo to the replicate command
• Add a link to the "Write to authorized_keys file"
• Add some things to the troubleshooting guide