Alerting: Notification channel http api enhancements (#16219)

Now returns uid in response to get notification channel by id.
Adds GET/PUT/DELETE support for notification channel by uid, 
  /api/alert-notifications/uid/:uid.
Break apart alerting and alert notification http api docs in two 
  pages and update documentation to make it up to date
  with current implementation.

Fixes #16012

pkg/services/sqlstore/alert_notification.go

@@ -130,6 +130,7 @@ func getAlertNotificationInternal(query *m.GetAlertNotificationsQuery, sess *DBS
 
 	sql.WriteString(`SELECT
 										alert_notification.id,
+										alert_notification.uid,
 										alert_notification.org_id,
 										alert_notification.name,
 										alert_notification.type,