docs(core): add standalone multi-canvas cities example

[ibgreen-openai]

Goal

Provide a complete, easy-to-run example of the multi-canvas API without mixing demo code into the core implementation review.

Changes

• Add a standalone TypeScript/Vite cities app rendering New York, London, Tokyo, and Sydney into four canvases owned by one Deck instance.
• Demonstrate independent views, controllers, basemaps, picking/tooltips, and per-view zoom widgets while sharing a device and layer stack.
• Link the example from the multi-canvas developer guide.

No core implementation changes are included in this PR.

Review stack

1. fix(core): position widgets relative to their canvas #10491 (merged) — widget/tooltip positioning preparation.
2. RFC/POC: Extend View system to support multi-canvas rendering #10229 — core multi-canvas implementation, rebased directly onto master.
3. This PR — standalone example, stacked on RFC/POC: Extend View system to support multi-canvas rendering #10229.

Validation

• ./node_modules/.bin/tsc -p modules/core/tsconfig.json --noEmit --pretty false
• yarn test-headless test/modules/core/lib/view-manager.spec.ts test/modules/core/lib/deck.spec.ts test/modules/core/lib/deck-renderer.spec.ts test/modules/core/passes/layers-pass.spec.ts test/modules/core/passes/pick-layers-pass.spec.ts test/modules/core/lib/deck-picker.spec.ts test/modules/core/lib/widget-manager.spec.ts test/modules/core/lib/tooltip.spec.ts test/modules/mapbox/mapbox-overlay.spec.ts --retry 1 --silent — 91 passing tests.
• ./node_modules/.bin/esbuild test/apps/multi-canvas-cities/app.ts --bundle --format=esm --platform=browser --external:@deck.gl-community/basemap-layers --outfile=/private/tmp/deck-gl-multi-canvas-cities.js
• yarn test-website — passes, including the monorepo build; existing dependency source-map and unrelated broken-anchor warnings remain.
• Full headless browser suite on the restacked branch: 825 tests passed across 201 files; one unrelated LoadingWidget test fails identically on pristine master.
• The inherited strict Mapbox integration regression passes repeatedly after deterministic first-draw synchronization.
• yarn build passes on current master plus the feature stack.
• Repository pre-commit lint/format checks and 21 Node smoke tests pass.

[coveralls]

Coverage is 83.166% — codex/multi-canvas-example into ib/multi-canvas-rfc. No base build found for ib/multi-canvas-rfc.