[Alerting V2] Attach related dashboards to a rule inline from the rule details page

## Summary

You can now attach, view, and remove related dashboards directly from the rule details page, through an inline "Attach related dashboards" popover with search — instead of reopening the rule's edit flyout. A rule with no linked dashboards shows a "No related Dashboards" state, and a rule whose linked dashboard was deleted still shows the broken link with a clear indicator rather than losing the reference.

**Why this needs docs:** This is the primary UI for dashboard artifacts on rules, which isn't mentioned anywhere in the current rule-authoring docs.

## Resources

- PR [#284481](https://github.com/elastic/kibana/pull/284481) — [Alerting V2] Inline add/edit for the Dashboards artifact widget on the rule details page
- Product issue: https://github.com/elastic/kibana/issues/279914
- Related: [#281751](https://github.com/elastic/kibana/pull/281751) — companion PR reworking the underlying artifact schema/validation these dashboard links depend on, tracked in [#8065](https://github.com/elastic/docs-content/issues/8065) (see that issue for the YAML schema field changes)

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 21–Aug 28 |
| **Feature status** | Technical preview |
| **Feature flag** | Requires the Alerting v2 preview to be enabled |

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

## Suggested edits

[Tags and runbooks in the experimental alerting system](https://www.elastic.co/docs/explore-analyze/alerting/experimental-alerting-system/rules/configure-rule-tags)
- **What's wrong:** This is the dedicated rule-artifacts page, but it only covers tags and runbooks — no mention of dashboards.
- **What to add:** Add a "Dashboards" subsection (see [#8065](https://github.com/elastic/docs-content/issues/8065), which flags the same gap) covering the UI workflow specifically: the inline "Attach related dashboards" popover with search, the "No related Dashboards" empty state, and the deleted-dashboard indicator. Applies from 9.6.0 (technical preview) and in serverless.