docs: add desktop raw backend WebGPU rendering example

[crowlKats]

Closes denoland/deno#35579.

That issue asks for a simple deno desktop raw backend example. This adds a dedicated WebGPU rendering page to the desktop docs.

What's here

New runtime/desktop/webgpu.md walks through drawing to a native window with WebGPU on the raw backend:

• Setup — desktop.backend: "raw" + unstable: ["webgpu"].
• A minimal example — open a window and clear it to a solid color (proves adapter → surface → context → present()).
• Drawing geometry — a hello-triangle render pipeline with a WGSL shader (no vertex buffers; positions baked in via @builtin(vertex_index)).
• Animating with a render loop — the triangle spins via a uniform buffer, driven by a self-scheduling setTimeout (raw has no requestAnimationFrame), with resize-reconfigure and a close → Deno.exit guard.
• Key details — request the adapter before getNativeWindow() (it throws otherwise), size/resize the surface, present() every frame, fresh texture per frame, and close() being downgraded to hide() once a surface is taken.

The examples are written fresh rather than reusing the snippet from the issue.

Wiring

• Added "WebGPU rendering" to the desktop sidebar (runtime/_data.ts) and the section list in runtime/desktop/index.md.
• Cross-linked into it from the raw section of backends.md and the "Native window handle" section of windows.md.

Testing

api_links_test.ts, sidebar_test.ts, and frontmatter_test.ts all pass.

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