Harden and clarify the documentation site

[kevinten10]

What changed

• Updated PostCSS, PostCSS CLI, and Autoprefixer; normalized the lockfile to the npm registry.
• Removed Docsy placeholder task pages and restored a real Chinese development-guides section.
• Corrected stale official documentation links and removed confirmed dead third-party images.
• Reworked the Chinese and English homepage copy to distinguish APIs, SDKs, adapters, current demos, and roadmap boundaries.
• Replaced the homepage background with a text-free AnyCap-generated hero visual, optimized from a 1.52 MB PNG to a 380 KB JPEG.

Why

The default branch had a high-severity PostCSS alert, retained template filler, referenced moved or deleted resources, and used absolute cross-cloud and production claims that exceeded the implementation evidence in the related repositories.

Validation

• npm audit: zero vulnerabilities
• npm test: 14 Node tests and Hugo 0.145 production render pass
• make link-check: 305 rendered-site links pass
• README local-link check
• Responsive browser QA at 1280 × 720, 768 × 1024, and 375 × 812
• Chinese and English homepage assets return 200; browser console has no errors
• git diff --check