Add example for refering to a file in alerts (#142)

This PR documents the second way to refer to alerts in DABs. Both alerts
have identical configurations and are in the same example to make it
clear that either mode is acceptable to use.

Tested using `databricks bundle validate` that this example works.

Author: shreyas-goenka

knowledge_base/alerts/resources/nyc_taxi_daily_revenue.alert.yml

@@ -48,3 +48,16 @@ resources:
         pause_status: "UNPAUSED"
         quartz_cron_schedule: "0 0 8 * * ?"  # Run daily at 8 AM UTC
         timezone_id: "UTC"
+
+    # This alert is configured identically to the above alert, but it refers to a .dbalert.json file
+    # instead of inlining the configuration in YAML. We recommend inlining the configuration in YAML
+    # when you need to productionize the alert across different target (dev, staging, prod) and
+    # using a .dbalert.json file when you have a single target alert that you author in the Databricks UI
+    # or using the databricks bundle generate alert command.
+    nyc_taxi_daily_revenue_as_file:
+      permissions:
+        - level: CAN_MANAGE
+          user_name: user@company.com
+      display_name: "NYC Taxi Daily Revenue Alert as File"
+      warehouse_id: ${var.warehouse_id}
+      file_path: ./nyc_taxi_daily_revenue_as_file.dbalert.json

knowledge_base/alerts/resources/nyc_taxi_daily_revenue_as_file.dbalert.json

@@ -0,0 +1,35 @@
+{
+  "custom_summary": "Alert when NYC Taxi daily revenue exceeds threshold",
+  "evaluation": {
+    "source": {
+      "name": "amount",
+      "display": "amount",
+      "aggregation": "MAX"
+    },
+    "comparison_operator": "GREATER_THAN",
+    "threshold": {
+      "value": {
+        "double_value": 1000000.0
+      }
+    },
+    "notification": {
+      "retrigger_seconds": 3600,
+      "notify_on_ok": false
+    }
+  },
+  "schedule": {
+    "quartz_cron_schedule": "0 0 8 * * ?",
+    "timezone_id": "UTC"
+  },
+  "query_lines": [
+    "SELECT",
+    "  to_date(tpep_pickup_datetime) as date,",
+    "  SUM(fare_amount) as amount",
+    "FROM",
+    "  `samples`.`nyctaxi`.`trips`",
+    "GROUP BY",
+    "  ALL",
+    "ORDER BY",
+    "  1 DESC"
+  ]
+}