docs: add documentation index

[leonbreukelman]

Summary

• Adds docs/index.md as the canonical documentation entry point.
• Links only to the existing README.md and leaves future docs topics as plain text to avoid dead links.

Build Arena provenance

• Selected by Build Arena weighted intake from a fresh, gate-passing Project Model snapshot.
• Selected finding: doc.index.missing.
• Patch gate accepted the final proposal for docs/index.md only.
• Fable reviewed the actual revised patch and approved it: good=true, safe=true, useful=true, required changes=[]

Verification

• test -e docs/index.md ✅
• relative link target check (../README.md) ✅
• uv run --extra dev python -m pytest -q ✅ 19 passed
• uv run --extra dev ruff check src/ tests/ ✅
• uv run --extra dev mypy src/ ✅
• git diff --cached --check ✅
• staged secret-shape scan ✅

No canonical checkout edits were made during Build Arena proposal generation; this PR packages the approved worktree proposal.

[Copilot]

Pull request overview

Adds a canonical documentation entry point under docs/ to centralize where documentation starts, while avoiding dead links by only linking to existing content.

Changes:

• Introduces docs/index.md as the documentation landing page.
• Links to the root README.md for installation/usage details and lists future doc topics as non-links.

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