Docs: Remove beta marker from Turbopack docs (vercel#71796)

Incorrectly mentions `(beta)`, it's been moved to stable in Next.js 15 👍

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docs/04-architecture/turbopack.mdx

@@ -3,7 +3,7 @@ title: Turbopack
 description: Turbopack is an incremental bundler optimized for JavaScript and TypeScript, written in Rust, and built into Next.js.
 ---
 
-[Turbopack](https://turbo.build/pack) (beta) is an incremental bundler optimized for JavaScript and TypeScript, written in Rust, and built into Next.js.
+[Turbopack](https://turbo.build/pack) is an incremental bundler optimized for JavaScript and TypeScript, written in Rust, and built into Next.js.
 
 ## Usage