Modernize documentation layout and fix outdated content

[cbyrohl]

Summary

• Redesign landing page with hero section (starfield background, feature cards, CTA buttons)
• Dark header and navigation tabs across all pages for consistent branding
• Consolidate three CSS files (gridview.css, code_select.css, mkdocstrings.css) into single extra.css
• Add dark/light mode palette toggle, breadcrumbs, and search enhancements to mkdocs config
• Migrate developer docs from Poetry to uv
• Fix outdated content: Python version info, units default documentation, cookbook link typo
• Remove dead polyfill.io script reference

Test plan

• Verify landing page renders correctly in both light and dark mode
• Verify navigation tabs and dark header appear on all docs pages
• Verify Simulations/Observations cards on install page display side-by-side
• Verify feature cards are readable in light mode
• Verify mobile responsive layout (cards stack vertically)
• Check developer.md references uv instead of poetry
• Build docs with mkdocs build to ensure no errors

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