[Alerting V2] Alerting privileges in the Roles UI are now grouped under "Alerting V2" and marked Experimental

## Summary

PR #288163 renames the Kibana feature privilege category shown in **Stack Management → Roles → Kibana privileges** from "Alerting" to "Alerting V2", to clarify that these privileges control the experimental (v2) alerting system rather than classic alerting. Each of the four features under that category — Rules, Alerts, Action Policies, and Execution history — now also shows a plain-text description starting with "Experimental. Controls access to …" in the privilege picker.

**Note on scope:** the PR description also claims it registers a new `alerting.alertingV2Overview` doc-links entry and opens overview docs from an info control next to the category, but neither change is present in the merged diff (only `privileges.ts`/`privileges.test.ts` changed) — so this issue only covers the confirmed rename + description text.

**Why this needs docs:** [Configure access to the experimental alerting system](https://www.elastic.co/docs/explore-analyze/alerting/experimental-alerting-system/get-started/configure-access) repeatedly instructs users to set privileges "under the Alerting category," which no longer matches the Roles UI now that the category reads "Alerting V2."

## Resources

- PR [#288163](https://github.com/elastic/kibana/pull/288163) — [Alerting v2] Mark Alerting V2 privileges as experimental in Roles UI
- Backport: PR [#288506](https://github.com/elastic/kibana/pull/288506) (9.5)

Screenshot from PR


## Availability

| Channel | Details |
|---------|---------|
| **Stack** | 9.5.3, 9.6.0 |
| **Serverless** | ~2026-09-08 (est.) |
| **Feature status** | Technical preview / Experimental |

## Suggested edits

[Configure access to the experimental alerting system](https://www.elastic.co/docs/explore-analyze/alerting/experimental-alerting-system/get-started/configure-access) > Throughout the page (intro, Quick reference table, Rules, View rule execution history, Triage alerts, Action policies)
- **What the docs say:** "Most privileges are set under the Alerting category in Kibana role management." The Quick reference table and each of the Rules / View rule execution history / Triage alerts / Action policies subsections repeat variants of "(under Alerting)".
- **What to add:** Update every "(under Alerting)" reference and the intro sentence to "Alerting V2" to match the renamed Roles UI category. Also worth a short note that each of these four privileges now shows an "Experimental" description in the Roles UI picker, consistent with the system's overall preview status. Applies from 9.5.3 and 9.6.0, and in serverless.

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*Filed via docs-quest-scanner triage, prompted by a direct PR review request.*