chore: move landing page to docs/ for GitHub Pages

[ditvor]

Summary

• Moves landing/index.html → docs/index.html
• Removes the now-empty landing/ directory
• Enables GitHub Pages to serve the site from docs/ on main with zero configuration

After merging

Enable GitHub Pages in repository Settings → Pages → Source: main branch, /docs folder.

Test plan

• Confirm docs/index.html exists with full content (645 lines)
• Confirm landing/ directory no longer exists
• After enabling GitHub Pages, verify the site loads at https://ditvor.github.io/pia

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