docs: Add v9.4 whats-new for alpha release

[chrisgervang]

Summary

• Expand docs/whats-new.md with 9.4 highlights: Controllers, GlobeView, Geo Layers, Performance

Test plan

• Review whats-new prose for accuracy
• Website builds cleanly with updated docs

🤖 Generated with Claude Code

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Note

Low Risk
Documentation-only change to the release notes page; no runtime or API code is modified.

Overview
Adds a full deck.gl v9.4 section to docs/whats-new.md with release date TBD, reorganizing and expanding what was previously a single views bullet into themed subsections.

Core performance documents instanced-layer picking without instancePickingColors, using shader builtins instance_index / gl_InstanceID.

Views and controllers groups GlobeView improvements (TerrainLayer, TerrainExtension, Tile3DLayer), the views parameters prop (including GlobeView back-face culling), and the doubleClickDragZoom controller gesture.

Package callouts add bullets for @deck.gl/geo-layers (TileLayer viewport-center prioritization, TerrainLayer zoomOffset), @deck.gl/arcgis (DeckRenderer / RenderNode vs externalRenderers), and @deck.gl/widgets (experimental ViewLayout + buildViewsFromViewLayout()).

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[coveralls]

coverage: 83.112%. remained the same — chr/9.4-docs into master