docs: Material for MkDocs site published to GitHub Pages

[cvetty]

Summary

Stands up a public documentation site for WhyGraph built with Material for MkDocs, published to GitHub Pages via a dedicated Actions workflow. Executes the round-5-reviewed plan in plans/docs-site-plan.md.

The defining constraint: the old README had drifted out of sync with the shipped product (dead render/serve commands, renamed MCP tools/prompts, a "confidence score" that doesn't exist). So every page is sourced from the live CLI --help and MCP registrations, not the README — and the README is trimmed to a pointer.

What's here

• mkdocs.yml — Material theme with the "fancy" treatment: card-grid landing, social (Open Graph) cards, instant nav + progress bar, light/dark toggle, code-copy, per-page "last updated" dates.
• 18 content pages under docs/ — Home, Getting Started (install/quickstart), User Guide (concepts/scanning/editors/MCP usage), Docker & Self-Hosting (docker/service), Reference (CLI/MCP/config/providers), Roadmap. All written in one locked voice (§5.1: second person, imperative, show-before-tell, asides in admonitions).
• deploy/service.md — the third-party-service model (real apps → whygraph-mcp for git analysis), with a Mermaid architecture diagram.
• pyproject.toml — new docs dependency group (uv sync --group docs).
• Makefile — make docs / make docs-build.
• .github/workflows/docs.yml — least-privilege Pages-via-Actions deploy (full-history checkout + Cairo/Pango for social cards).
• README.md — trimmed to pitch + Quickstart + "Full documentation →" link.

Verification

• uv run mkdocs build --strict → clean (no broken links / missing nav).
• Dead-surface grep clean (no render/serve/whygraph_search/--llm-recent/claude-desktop/confidence).
• CLI/MCP reference matches live --help + mcp/*.py registrations.
• CI=true build degrades gracefully without Cairo (cards generate in CI, skip locally).
• Wheel still excludes docs/; ci-code-checks.yml / cd-deploy-whygraph.yml untouched.

⚠️ One-time maintainer step before the site goes live

GitHub Pages must be set to Settings → Pages → Source → GitHub Actions (or gh api -X POST repos/mtrdesign/whygraph/pages -f build_type=workflow). A committed workflow can't set this for itself on the first run. Until it's flipped, the build job goes green but deploy can't publish.