docs: refine homepage and code examples

[ZhiXiao-Lin]

Summary

• rebuild the homepage with concrete A3S Code copy, a keyboard-accessible 3D runtime stack, and a Python Code Hike walkthrough
• show the official macOS/Linux and Windows one-click installers on the homepage and in both guide languages
• replace remaining documentation code fences with the shared Code Hike renderer and remove the TUI/Web component catalog

Validation

• npm run format
• npm run lint
• npm run build
• npm run check:site
• desktop and 390px browser interaction checks for installer tabs, animated CTA, 3D layers, tutorial scrolling, code copy/wrap/expand, and console errors