docs: add llms.txt for the documentation site

[sdiebolt]

Summary

Add an llms.txt (and optionally llms-full.txt) at the docs site root so LLM agents have a curated, machine-parseable entry point to ConfUSIus's documentation.

Background

llms.txt is a proposed standard (Jeremy Howard / Answer.AI, Sep 2024) for giving LLMs a concise entry point to a site's docs. Adopted by FastAPI, Next.js, Vite, Vue, Cloudflare, Mintlify, and many others.

It is a single Markdown file at the site root with a fixed structure (H1, blockquote, optional prose, then H2-delimited link lists). LLMs use it to discover what's available; humans can read it too.

Proposal

1. Hand-curate docs/llms.txt in the repo, with sections like:
   • ## User Guide — links to the main user-guide pages
   • ## API Reference — links to the mkdocstrings-rendered API pages
   • ## Examples — links to the Jupytext-built example gallery
   • ## Changelog — link to the changelog
   • ## Optional — links to external references (BIDS spec, xarray docs, etc.)
2. Submit the deployed URL (https://confusius.tools/llms.txt) to the llmstxt.site directory for discoverability (one-click).
3. (Optional follow-up) Generate llms-full.txt in CI by walking site/**/*.html, converting to markdown, and concatenating — gives agents the full ~45-page context in one fetch.

Why

• ~45 pages of docs (user guide, API reference, examples) are currently invisible to LLMs unless they browse
• A clean llms.txt makes the package discoverable and usable by Claude Code, Codex, Gemini CLI, Goose, Cursor, Copilot, etc.
• Deploys automatically via the existing .github/workflows/docs.yml deploy-docs job — no new infra
• Low effort, high signal

Effort

Low. The hand-curated file is ~30 min; the generator + CI hook is a follow-up.