Add second exemplar community layout

## Summary
Only one community layout exists. A second complete example helps new contributors.

## Suggested layouts
- `comparison-table` — two columns with headings
- `timeline-horizontal` — steps across the slide
- `team-grid` — photo + name cards

## Requirements
Full bundle under `community//`:
- `slide.config.ts`
- `thumbnail.svg`
- `README.md`
- Optional `Slide.tsx`

## Files
- `community/` (new directory)
- `community/README.md`

Start by inspecting the existing community layout and community/README.md to mirror the project's exemplar structure. Choose one of the suggested layouts and add a complete community// bundle with slide.config.ts, thumbnail.svg, README.md, and an optional Slide.tsx; the work is done when the new example matches those requirements.

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