Document the full Lightcone research stack and simplify onboarding

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The documentation was centered on the CLI, leaving researchers without a clear introduction to the full stack or a consistent setup path. This refactor organizes the site around installing the tools, running a first analysis, working with an agent, and sharing results, with technical reference material behind those guides.

Changes

• Introduce the Lightcone CLI and agent plugin together, with ASTRA explicitly identified as an external specification and toolset.
• Simplify installation and replace the long tutorial with a complete, self-contained estimator comparison.
• Add agent, reporting, and sharing guides; update compute, cluster, troubleshooting, and contributor documentation.
• Preserve existing page URLs, including a migration page for the former venue reference.
• Match the Lightcone website's typography, parchment and gold palette, engraving, and editorial layout, with responsive navigation and light/dark themes.

Compatibility

These docs intentionally target the upcoming explicit compute workflow. Installation pins public CLI commit 835de9e7c2df726722ffff8c6863d6f9b16ee577 so the documented commands are available before a PyPI release. The agent guide explains the gap between that preview and the plugin's earlier CLI target. Slurm configuration examples are illustrative and have not been tested on a live facility.

Validation

• uv run zensical build --clean --strict and git diff --check pass.
• Checked 2,889 local links and fragments across 38 generated HTML pages.
• Executed the tutorial against an isolated installation of the pinned preview: ASTRA validation, correct mean/median results, provenance and clean Git state, repeat-run reuse, and allocation shutdown all passed.
• Browser checks passed for search, plugin tabs, dark mode, mobile navigation, and 24 responsive page/viewport combinations, with no JavaScript errors or page overflow.

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Closing: the Zensical site was removed in #4 ahead of the Fumadocs rebuild. The branch stays available if any of this content needs carrying over.