Nav: Update Observability section nav to phase 2 (#112806)

* Move up a level the knowledge graph children

* Remove unused frontend id

* Add special id to plugin pages

pkg/services/navtree/models.go

@@ -26,7 +26,6 @@ const (
 	WeightCloudServiceProviders
 	WeightInfrastructure
 	WeightApplication
-	WeightFrontend
 	WeightAsserts
 	WeightDataConnections
 	WeightApps
@@ -48,7 +47,6 @@ const (
 	NavIDAlerting             = "alerting"
 	NavIDObservability        = "observability"
 	NavIDInfrastructure       = "infrastructure"
-	NavIDFrontend             = "frontend"
 	NavIDReporting            = "reports"
 	NavIDApps                 = "apps"
 	NavIDCfgGeneral           = "cfg/general"