[Doc] Document import support for databricks_notification_destination (#4276)

## Changes
<!-- Summary of your changes that are easy to understand -->

resolves #4275

## Tests
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- [ ] `make test` run locally
- [x] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
- [ ] relevant acceptance tests are passing
- [ ] using Go SDK

Author: alexott

docs/resources/notification_destination.md

@@ -98,3 +98,11 @@ In addition to all arguments above, the following attributes are exported:
 
 * `id` - The unique ID of the Notification Destination.
 * `destination_type` - the type of Notification Destination.
+
+## Import
+
+This resource can be imported by notification ID:
+
+```bash
+terraform import databricks_notification_destination.this <notification-id>
+```