Geo: document archived PostgreSQL replication as an option when using DR

This is one thing @stanhu mentioned during his last office hours:

Instead of real streaming replication, we use archived WAL replication for our gitlab.com DR site (using Wal-E). By doing this, the Geo secondary node will never cause stress on the primary node, because when a streaming replication slot would be used, the slot could clog up things when the secondary node is lagging far behind.