Export and recreate by-value Vega panels with Dashboards as Code

## Summary

When the vega.standaloneEmbeddable feature flag is on, by-value Vega panels register a server schema and take part in Dashboards as Code export, import, and OpenAPI. Legacy Vega visualizations from the Visualize editor or the library stay unsupported. The flag is off by default, and the release note was skipped. The programmatic dashboards page lists supported panel types and still treats types without a schema, such as Maps, as write errors.

**Why this needs docs:** Dashboards as Code docs list supported panel types, and Vega is now a schema-backed type only when a default-off flag is enabled.

## Resources

- PR [#283508](https://github.com/elastic/kibana/pull/283508) — [Vega as Code] Support by-value Vega embeddable panels in Dashboards as Code endpoints
- Product issue: https://github.com/elastic/kibana/issues/279553

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Sep 7–Sep 11 |
| **Feature flag** | None — active by default |

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

## Suggested edits

[Create dashboards programmatically](https://www.elastic.co/docs/explore-analyze/dashboards/create-dashboards-programmatically) > Dashboards API
- **What the docs say:** The API supports any panel type that has a defined schema: Visualizations, Discover sessions, Markdown, Image, Controls, SLO panels, Synthetics panels, Links, APM service map, and Machine learning and AIOps panels. Panel types without a defined schema, such as Maps, aren't supported yet and return an error on write.
- **What to add:** If the flag is treated as a supported 9.6 capability, add by-value Vega (type vega, spec.format json|hjson plus spec.value) to the supported panel types, and say library or Visualize-editor Vega panels are still dropped. Do not list Vega as generally available while the flag stays off. Applies from 9.6.0 and in serverless when vega.standaloneEmbeddable is enabled.