Choose solid or gradient fills for Lens area charts

## Summary

Lens area charts now have a Fill control in the Style panel with Solid and Gradient choices, while Solid remains the default for new and existing Lens charts. The Visualizations API exposes the same choice as `styling.areas.fill`; Kibana also changes the default Vega area mark to a gradient unless a color encoding overrides it with a solid fill.

**Why this needs docs:** The diff adds a visible Lens styling control and changes Vega's default area rendering, but the published area-chart settings and Vega default-theme sections do not describe either behavior.

## Resources

- PR [#277546](https://github.com/elastic/kibana/pull/277546) — [Lens][XY] Add gradient style option to area charts

Screenshots from PR


## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 17–Aug 21 |
| **Feature flag** | None — active by default |

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## Suggested edits

[Build area charts with Kibana](https://www.elastic.co/docs/explore-analyze/visualize/charts/area-charts) > Area chart settings > General layout > Style > Appearance
- **What the docs say:** The Appearance list documents area fill opacity, point visibility, and line interpolation, and the API examples set only `styling.areas.fill_opacity`.
- **What to add:** Add the Fill control and its Solid and Gradient options, noting that Solid is the default and existing saved Lens area charts remain solid; also show or explain the matching `styling.areas.fill` property in the page's API coverage. Applies from 9.6.0 and in serverless.

[Custom visualizations with Vega](https://www.elastic.co/docs/explore-analyze/visualize/custom-visualizations-with-vega) > Vega reference > Default theme to match Kibana
- **What the docs say:** The section says Kibana sets a default color for each Vega mark type and that authors can override it with stroke, fill, or color values.
- **What to add:** Clarify that an area mark with no explicit fill now uses Kibana's default gradient, while a Vega or Vega-Lite color encoding takes precedence and renders a solid series color. Applies from 9.6.0 and in serverless.