Documents ML anomaly detection and Stack Management alerts on the Observability Alerts page (#6848)

<!--
Thank you for contributing to the Elastic Docs! 🎉
Use this template to help us efficiently review your contribution.
-->

## Summary
<!--
Describe what your PR changes or improves.  
If your PR fixes an issue, link it here. If your PR does not fix an
issue, describe the reason you are making the change.
-->

Fixes #6602.

Updates the Observability Alerting overview to clarify which rule types
surface alerts on the **Alerts** page. Previously, the page described
how to view and filter alerts but gave no indication of which alert
consumers were included, leaving users who create ML anomaly detection
or Stack Management rules without any explanation for why those alerts
might not appear.

## Changes

`solutions/observability/incident-management/alerting.md`

- Adds a bullet list under the "[Alerts
page](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/6848/solutions/observability/incident-management/alerting#observability-alerting-alerts-page)"
section enumerating the three supported alert type categories:
Observability rules (APM, Logs, Metrics, Uptime, SLOs), ML anomaly
detection rules, and Stack Management rules.
- Tags the ML and Stack Management list items with `{applies_to} stack:
ga 9.5` to reflect that these alert types became visible on the
Observability Alerts page in 9.5.0. Serverless inherits the page-level
`serverless: ga` tag.
- Adds a note that ML anomaly detection and Stack Management alerts are
visible only to users with the corresponding feature privileges, as
access is governed by backend RBAC.

## Generative AI disclosure
<!--
To help us ensure compliance with the Elastic open source and
documentation guidelines, please answer the following:
-->
1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [x] Yes - Cursor + Claude
- [ ] No  
<!--
2. If you answered "Yes" to the previous question, please specify the
tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used:
-->

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: nastasha-solomon

solutions/observability/incident-management/alerting.md

@@ -33,6 +33,16 @@ On the **Alerts** page, the Alerts table provides a snapshot of alerts occurring
 :screenshot:
 :::
 
+The **Alerts** page displays alerts from the following rule types:
+
+* Observability rules, including APM, Logs, Metrics, Uptime, and SLO rules
+* {applies_to}`stack: ga 9.5+` {{ml}} anomaly detection rules
+* {applies_to}`stack: ga 9.5+` {{stack-manage-app}} rules
+
+::::{note}
+ML anomaly detection and Stack Management alerts are visible only to users with the corresponding feature privileges, as access is controlled by role-based access control (RBAC).
+::::
+
 You can filter this table by alert status or time period, customize the visible columns, and search for specific alerts (for example, alerts related to a specific service or environment) using KQL. Select **View alert detail** from the **More actions** menu ![action menu](/solutions/images/serverless-boxesHorizontal.svg ""), or click the Reason link for any alert to [view alert](/solutions/observability/incident-management/view-alerts.md) in detail, and you can then either **View in app** or **View rule details**.