docs: add system routes description (#901)

* docs: add system routes description

---------

Signed-off-by: Mauritz Uphoff <mauritz.uphoff@stackit.cloud>

stackit/internal/services/iaasalpha/routingtable/shared/shared.go

@@ -205,7 +205,8 @@ func RoutingTableResponseAttributes() map[string]schema.Attribute {
 			Computed:    true,
 		},
 		"system_routes": schema.BoolAttribute{
-			Computed: true,
+			Description: "This controls whether the routes for project-to-project communication are created automatically or not.",
+			Computed:    true,
 		},
 		"created_at": schema.StringAttribute{
 			Description: "Date-time when the routing table was created",