fix: document Next.js automatic JSX requirements - #381

Summary

• replace the stale SPA-only automatic JSX limitation with current React SPA and Next.js support
• document the required @generaltranslation/compiler installation and Next.js Babel compiler setup
• clarify that Next.js automatic JSX injection requires Webpack and is unavailable with Turbopack or SWC

Verification

• installed and exercised gt-next@11.0.10 and @generaltranslation/compiler@1.3.32
• built a Next.js 16.2.6 fixture successfully with next build --webpack
• confirmed Turbopack emits the published incompatibility warning
• pnpm build:content
• node --experimental-strip-types scripts/validate-links.ts