feat: embed gemara-react live demo in the website

[jpower432]

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

---

Formatting done by 🤖

[jpower432]

cc @eddie-knight