Merge branch 'docs/gcp-sql-read-replica-link' into 'master'

Geo - Add link to Google Cloud SQL read replica instructions

See merge request gitlab-org/gitlab!62515

doc/administration/geo/setup/external_database.md

@@ -64,6 +64,7 @@ cloud providers:
 
 - Amazon RDS - [Creating a Read Replica](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.html#USER_ReadRepl.Create)
 - Azure Database for PostgreSQL - [Create and manage read replicas in Azure Database for PostgreSQL](https://docs.microsoft.com/en-us/azure/postgresql/howto-read-replicas-portal)
+- Google Cloud SQL - [Creating read replicas](https://cloud.google.com/sql/docs/postgres/replication/create-replica)
 
 Once your read-only replica is set up, you can skip to [configure you secondary application node](#configure-secondary-application-nodes-to-use-the-external-read-replica).