Switch the Discover table between Table and JSON view

## Summary

Discover adds a View mode control that switches the documents table between Table and JSON. JSON mode can hide nulls and wrap long lines. The control is behind `discover.dataTableJsonView` and is off by default. Customize the Discover view already documents Display options such as density, row height, and sample size. It does not mention View mode.

**Why this needs docs:** This is new table UI, but the View mode control is hidden until the feature flag is on.

## Resources

- PR [#285912](https://github.com/elastic/kibana/pull/285912) — [Discover][Table] Add JSON view UI toggle
- Product issue: https://github.com/elastic/kibana/issues/281552

Screenshots from PR


## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 31–Sep 4 |
| **Feature flag** | None — active by default |

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## Suggested edits

[Customize the Discover view](https://www.elastic.co/docs/explore-analyze/discover/document-explorer) > Modify the document table
- **What the docs say:** Customize the appearance of the document table from Display options. The page covers density, row height, sample size, sorting, and rows per page.
- **What to add:** When `discover.dataTableJsonView` is on, add View mode with Table and JSON. Document Hide nulls and Wrap lines for JSON mode. Do not publish this as generally available while the flag is off. Applies from 9.6.0 and in serverless when the flag is enabled.