docs(alerting): simplify Intro to Grafana Alerting docs (#106944)

* docs(alerting): improve `Intro > Alert rule evaluation` docs

* Update Introduction to Grafana Alerting

* Simplify `Intro > Alert rules` and related docs

* minor copy change phrasing GMA and DS differences

* fix vale error

docs/sources/shared/alerts/configure-alert-rule-name.md

@@ -5,8 +5,6 @@ labels:
 title: 'Set alert rule name'
 ---
 
-## Set alert rule name
-
 1. Click **Alerts & IRM** -> **Alert rules** -> **+ New alert rule**.
 1. Enter a name to identify your alert rule.
 

docs/sources/shared/alerts/configure-notification-message.md

@@ -5,8 +5,6 @@ labels:
 title: 'Configure notification message'
 ---
 
-## Configure notification message
-
 Use [annotations](ref:shared-annotations) to add information to alert messages that can help respond to the alert.
 
 Annotations are included by default in notification messages, and can use text or [templates](ref:shared-alert-rule-template) to display dynamic data from queries.

docs/sources/shared/alerts/note-prometheus-ds-rules.md

@@ -1,10 +0,0 @@
----
-labels:
-  products:
-    - oss
-title: 'Note Prometheus data source-managed rules'
----
-
-> Rules from a Prometheus data source appear in the **Data source-managed** section of the **Alert rules** page when [Manage alerts via Alerting UI](ref:shared-configure-prometheus-data-source-alerting) is enabled.
->
-> However, Grafana can only create and edit data source-managed rules for Mimir and Loki, not for a Prometheus instance.