Defer loading of dashboard panels that are out of view

## Summary

This pull request restores labs:dashboard:deferBelowFold. Off-screen panels now wait to fetch data until they enter the viewport, using IntersectionObserver. Screenshot and reporting mode still loads every panel. The advanced settings page still says that from 9.4.3 the setting has no effect and that deferred loading can break dashboards.

**Why this needs docs:** Published advanced settings still claim the lab setting is inert after 9.4.3, which is now wrong on 9.6.0 and 9.5.2.

## Resources

- PR [#284004](https://github.com/elastic/kibana/pull/284004) — [dashboard] defer panel loading when panels are not visible
- Product issue: https://github.com/elastic/kibana/issues/271236

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.5.2, v9.6.0 |
| **Serverless** | N/A |
| **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

[Kibana advanced settings](https://www.elastic.co/docs/reference/kibana/advanced-settings) > Presentation Labs — labs:dashboard:deferBelowFold
- **What the docs say:** Enables deferred loading of dashboard panels below the fold. Note: From version 9.4.3, all panels load immediately regardless of this setting. Deferred loading of below-the-fold panels is disabled because enabling it can cause some dashboards to fail to load. Default: false.
- **What to add:** Keep the 9.4.3 note for versions that still ignore the setting. Then document that from 9.5.2 and 9.6.0 the setting works again: when it is on, panels fetch only after they become visible, and screenshot mode still fetches every panel. Do not put a version number in the same sentence as an applies_to badge. Stack only, from 9.6.0 (not available in serverless).