docs(next): add metadata translation example and link pluralization from quickstarts

[moss-bryophyta]

DX Feedback Loop — Run 3 (2026-03-22)

What triggered this

A naive sub-agent building a Next.js app with metadata translation (page titles, descriptions, OG tags) could not find any documentation on how to use getGT() inside generateMetadata(). The agent had to read .d.ts files to figure out the pattern.

Wrong approach the agent might have tried

• Using <T> inside metadata (impossible — metadata is a plain object, not JSX)
• Using useGT() hook inside generateMetadata() (wrong — it's an async server function, not a React component)

What this PR does

1. Adds a metadata translation example to the getGT API reference page showing the generateMetadata() pattern with titles, descriptions, and OG tags
2. Adds pluralization and metadata links to the Next.js quickstart "Next steps" section so developers can discover <Plural> and the metadata pattern
3. Adds pluralization link to the React quickstart "Next steps" section

Correct pattern

import { getGT } from 'gt-next/server';

export async function generateMetadata(): Promise<Metadata> {
  const gt = await getGT();
  return {
    title: gt('My App'),
    description: gt('A fast, modern web application'),
    openGraph: {
      title: gt('My App'),
      description: gt('A fast, modern web application'),
    },
  };
}