docs: add installation guide page for GitHub Pages
Adds docs/install.html — a visually polished, bilingual (EN/DE) installation guide matching the index.html design system. Features step-by-step layout with numbered steps, copy buttons, tabbed Option A/B install paths, troubleshooting accordion, and dark mode support. Updates docs/index.html with a secondary "Installation Guide" CTA in the hero section and updated setup-note link pointing to the new page instead of the raw GitHub markdown file. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in: