Docs: Change header levels of last three sections (#23587)

The last three sections of this document are not rendered on https://grafana.com/docs/grafana/latest/alerting/notifications 
It seems like in all other instances (other .md docs of this project), level one headers are only used at the top of the page, so I assume using level two headers will make them show. Level two headers do also match the overall structure of the documentation.

docs/sources/alerting/notifications.md

@@ -208,7 +208,7 @@ Threema | `threema` | yes, external only | no
 VictorOps | `victorops` | yes, external only | no
 VictorOps | `victorops` | yes, external only | no
 Webhook | `webhook` | yes, external only | yes
 Webhook | `webhook` | yes, external only | yes
 
 
-# Enable images in notifications {#external-image-store}
+## Enable images in notifications {#external-image-store}
 
 
 Grafana can render the panel associated with the alert rule as a PNG image and include that in the notification. Read more about the requirements and how to configure
 Grafana can render the panel associated with the alert rule as a PNG image and include that in the notification. Read more about the requirements and how to configure
 [image rendering]({{< relref "../administration/image_rendering/" >}}).
 [image rendering]({{< relref "../administration/image_rendering/" >}}).
@@ -220,7 +220,7 @@ Be aware that some notifiers requires public access to the image to be able to i
 
 
 Notification services which need public image access are marked as 'external only'.
 Notification services which need public image access are marked as 'external only'.
 
 
-# Use alert rule tags in notifications {#alert-rule-tags}
+## Use alert rule tags in notifications {#alert-rule-tags}
 
 
 > Only available in Grafana v6.3+.
 > Only available in Grafana v6.3+.
 
 
@@ -230,7 +230,7 @@ It currently supports only the Prometheus Alertmanager notifier.
 
 
  This is an optional feature. You can get notifications without using alert rule tags.
  This is an optional feature. You can get notifications without using alert rule tags.
 
 
-# Configure the link back to Grafana from alert notifications
+## Configure the link back to Grafana from alert notifications
 
 
 All alert notifications contain a link back to the triggered alert in the Grafana instance.
 All alert notifications contain a link back to the triggered alert in the Grafana instance.
 This URL is based on the [domain]({{< relref "../installation/configuration/#domain" >}}) setting in Grafana.
 This URL is based on the [domain]({{< relref "../installation/configuration/#domain" >}}) setting in Grafana.