[Alerting V2] Rule artifacts (runbooks and dashboards) now use structured per-type fields, with better handling of deleted dashboards

## Summary

Rule artifacts (runbooks and linked dashboards) now use structured, per-type fields instead of a single string value:

| Before | After |
|---|---|
| `artifacts[].value: string` | `artifacts[].data: object` |

**Validation is now per artifact type:** a runbook needs non-blank `content` (up to 50,000 characters); a dashboard needs a `dashboardId`; any other field defaults to a 1,024-character cap.

**Also new:** a rule whose linked dashboard was deleted now shows a "Dashboard deleted" indicator and survives saving, instead of being silently dropped.

This is a breaking change to the YAML `artifacts[].value` field specifically — existing saved rules are migrated automatically on read, but any docs or examples using `value` are now wrong.

**Why this needs docs:** The YAML schema reference documents the exact field this PR removed, and neither existing rule-authoring page mentions dashboards as an artifact type at all.

## Resources

- PR [#281751](https://github.com/elastic/kibana/pull/281751) — [Alerting V2] Change artifacts schema
- Product issue: https://github.com/elastic/rna-program/issues/764
- Related: [#284481](https://github.com/elastic/kibana/pull/284481) — companion PR adding the inline "Attach related dashboards" UI (tracked separately)

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 12–Aug 19 |
| **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

### 1. [YAML rule schema reference](https://www.elastic.co/docs/explore-analyze/alerting/experimental-alerting-system/rules/yaml-rule-schema-reference) > Artifact fields — breaking change
- **What's wrong:** Documents `artifacts[].value` (string, markdown content) as the field for artifact content. That field no longer exists.
- **What to add:** Replace `artifacts[].value` with `artifacts[].data` (object), and document the per-type shape:
- `runbook`: `{ content: string }` — required, non-blank, max 50,000 characters
- `dashboard`: `{ dashboardId: string }` — required
- any other/unregistered type: unvalidated `data`, other fields default to a 1,024-character cap
Update the `type` field's example to list both `runbook` and `dashboard`. Applies from 9.6.0 (technical preview) and in serverless.

### 2. [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 page for rule artifacts, but it only covers tags and runbooks — dashboards aren't mentioned as an artifact type anywhere, despite having their own UI on the rule details page.
- **What to add:** Add a "Dashboards" subsection alongside Tags and Runbooks (or retitle the page, at the writer's discretion) covering: how to link a dashboard for investigation context, and that a rule whose linked dashboard was deleted shows a "Dashboard deleted" indicator rather than losing the link silently. Applies from 9.6.0 (technical preview) and in serverless.