Add nav entries for Geo

@fzimmer @rnienaber @ashmckenzie

How about this for an MVC? It puts Geo "in brackets" rather than calling these things "Geo".

• Click here: http://docs-add-geo-to-navigation.178.62.207.141.xip.io/ce/administration/geo/disaster_recovery/ to see how the navigation behaves for Disaster Recovery.
• Click here: http://docs-add-geo-to-navigation.178.62.207.141.xip.io/ce/administration/geo/replication/ to see how navigation behaves for Replication.