feat: add GitHub Pages site with CI deployment

[AndrewAltimit]

Summary

• Add a complete static GitHub Pages site in site/ showcasing the ITK framework and NMS Cockpit Video project
• Four pages: landing, architecture deep-dive, NMS project showcase, and technical docs with sidebar TOC
• Modern dark theme (Vercel/Linear-inspired) with responsive design, scroll animations, tabs, accordion, and copy-to-clipboard
• Pure HTML/CSS/JS -- no build tools, no frameworks, only Google Fonts as external dependency
• Add deploy-pages job to main-ci.yml that auto-deploys to GitHub Pages on push to main
• Add .nojekyll at repo root to disable Jekyll processing

Test plan

• Open each HTML file locally in a browser to verify rendering
• Test responsive breakpoints (320px, 768px, 1024px, 1440px) with browser dev tools
• Verify all internal links between pages work
• Verify interactive elements: mobile nav toggle, tabs, accordion, copy buttons, scroll animations, TOC tracking
• Confirm .nojekyll is at repo root
• After merge, verify GitHub Pages deployment succeeds in Actions
• Set repo Settings > Pages source to "GitHub Actions"

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