chore: correct node integration docs to match published render() signature

[janechu]

What

Rewrite the three framework examples and the API Reference table in docs/guide/integrations/node.md to match the actual signature of render() / renderStream() exported from @microsoft/webui.

Why

I'm integrating WebUI into a non-WebUI Node host, and the published Node integration page is the first thing a new adopter reads. The page currently teaches:

render(protocol, JSON.stringify(state), entry, requestPath, onChunk)

but packages/webui/src/index.ts actually exports:

render(protocol: Buffer, state: object | string, options?: RenderOptions): string
renderStream(protocol: Buffer, state: object | string, onChunk: (html: string) => void, options?: RenderOptions): void

Copying any of the three examples into a fresh project does not work, in three different ways:

1. entry, requestPath, and plugin are properties on a RenderOptions object, not positional args.
2. state accepts an object directly, so JSON.stringify is unnecessary boilerplate that suggests the API requires it.
3. render returns the full HTML as a string; only renderStream invokes a chunk callback. The Bun and Deno examples collect chunks into an array and .join('') them, which is doubly confusing because that's exactly what render already does internally.

How I tested

packages/webui/src/index.ts is the source of truth for the exported signatures. The three rewritten examples match the exported types and use the correct entry point (renderStream where streaming is desired, render for accumulate-then-respond).

Notes

• Branch is users/janechu/fix-node-integration-docs per my host repo's agent-driven branch convention. Happy to rename to janechu/fix-node-integration-docs if you prefer.
• Sibling of chore: correct rust integration docs to match published crate #295 (Rust integration docs fix). Filing separately because the changes are independent.