Add documentation site

[taddyb]

Summary

• Add zensical-powered documentation site with four pages: landing, getting started, benchmarks, and API reference
• Add docs optional dependency group (zensical, mkdocstrings[python]) to pyproject.toml
• Add GitHub Actions workflow for deploying to GitHub Pages on push to master
• Hand-written API docs for PyO3-exported Rust functions since mkdocstrings cannot introspect .so modules

Pages

• index.md — Install, quick example, coverage methods, supported statistics, requirements
• getting-started.md — Data loading, computing stats, coverage methods, nodata handling, memory, CRS, saving
• benchmarks.md — CONUS-scale timing (225K basins), accuracy validation, environment
• api-reference.md — xarray accessors (mkdocstrings), low-level API (hand-written signatures), statistics table

Test plan

• cd extractrs && uv sync --extra docs && zensical serve renders all 4 pages
• mkdocstrings renders Dataset/DataArray accessor docstrings
• All internal links resolve
• Light/dark mode toggle works
• zensical build --clean produces site/ with no errors

🤖 Generated with Claude Code