Link external postgresSQL setup page to setup page for Geo

doc/administration/geo/setup/index.md

@@ -30,7 +30,9 @@ a single-node Geo site or a multi-node Geo site.
 
 If both Geo sites are based on the [1K reference architecture](../../reference_architectures/1k_users.md):
 
-1. [Set up the database replication](database.md) (`primary (read-write) <-> secondary (read-only)` topology).
+1. Set up the database replication based on your choice of PostgreSQL instances (`primary (read-write) <-> secondary (read-only)` topology):
+   - [Using Omnibus PostgreSQL instances](database.md) .
+   - [Using external PostgreSQL instances](external_database.md)
 1. [Configure GitLab](../replication/configuration.md) to set the **primary** and **secondary** sites.
 1. Recommended: [Configure unified URLs](../secondary_proxy/index.md) to use a single, unified URL for all Geo sites.
 1. Optional: [Configure Object storage replication](../../object_storage.md)