Add FAQ section and translations for multiple languages

[Creylay]

Summary

Adds the FAQ section (#faq) to the site, based on the dashai-faq-medios.html design.

• 28 questions across 6 categories: General, Installation, Data & privacy, Usage & models, Extensibility and Community.
• Search box, "Open all" toggle, and a sidebar that highlights the category in view.
• Direct links to each category (e.g. #faq-privacy).
• Content in all 5 languages (es, en, pt, de, zh).
• "FAQ" link in the navbar and footer.

Testing

1. Run pnpm dev and open localhost:3000/#faq.
2. Try the search box (e.g. "gpu"), the "Open all" toggle and the sidebar links.
3. Switch languages from the navbar.