feat: embed gemara-react live demo in the website

## Problem / Motivation

The website has no interactive demo. `@gemara/react` provides headless renderers for Gemara artifacts. Showcasing them on the site makes the project's capabilities immediately tangible.

## Proposed Solution

Add a demo page (e.g. `sdk/demo.md` or a standalone `demo/` section) that embeds a live or pre-rendered example using `gemara-react`.

## Acceptance Criteria

- [ ] Demo page renders at least one Gemara artifact using `@gemara/react`
- [ ] Page is linked from the nav and the home page
- [ ] Build still passes with no new generated files committed

## Related

- `sdk/` folder — natural home for SDK-level demos
- `@gemara/react` — `gemaraproj/gemara-react`

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> Formatting done by 🤖

Start by inspecting the existing site structure, the sdk/ folder, and the navigation and home-page entry points. Review how @gemara/react is consumed, then determine whether sdk/demo.md or a standalone demo/ section fits the site. Done means a page renders at least one Gemara artifact, is linked from navigation and home, and the build passes without committing generated files.

Written by the indexing model from the issue text.