[Alerting V2] [M2] Document the Dashboards artifacts subsection on rule details

## Summary

Adds a 'Dashboards' subsection under an Artifacts accordion on the Alerting v2 rule details Overview tab (Alert-mode rules only): view linked dashboard artifacts with resolved titles and open-in-new-tab links, remove a linked dashboard with confirmation, see warning states for missing/deleted dashboards, and manage links via a 'Manage linked dashboards' action that opens the Compose Discover edit flyout.

**Why this needs docs:** The drafted docs only cover one artifact type (runbooks) — yaml-rule-schema-reference.md's Artifact fields table documents artifacts[].type generically with a runbook example, and view-manage-rules.md's rule-details-page section lists only Overview/Conditions/Runbook tabs with no mention of an Artifacts accordion or Dashboards subsection — so this new UI surface for linked dashboards has no home in the current draft.

## Resources

- PR [#274633](https://github.com/elastic/kibana/pull/274633) — [Alertingv2] Rule Details Page - artifacts related dashboards

Screenshots from PR


## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.5.0 |
| **Serverless** | Jun 29–Jul 3 |
| **Feature status** | preview |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @nastasha-solomon*

## Suggested edits

[View and manage rules in the experimental alerting system](https://www.elastic.co/docs/explore-analyze/alerting/kibana-alerting-experimental/rules/view-manage-rules) > Review rule configuration and activity
- **What the docs say:** "Overview (Alert mode only): Shows an alert activity timeline for each series the rule tracks... Runbook: If the rule has an investigation guide, it appears here." No mention of an Artifacts accordion or any Dashboards subsection on the Overview tab.
- **What to add:** This page (still in open docs-content PR #6523) describes the Overview tab's alert activity timeline and a separate Runbook tab, but omits the Artifacts accordion and its Dashboards subsection: viewing linked dashboards, handling missing/deleted dashboard warning states, and managing links via the Compose Discover edit flyout. A user relying on this page wouldn't know linked dashboards exist on the rule details page. Applies from 9.5.0 (experimental) and in serverless (experimental).