Add GitHub Pages documentation site

[YuminosukeSato]

Summary

• Add a MkDocs Material documentation site deployed to GitHub Pages
• Tutorial page mirrors the R CausalImpact site structure
• Plot images are generated in CI from a deterministic script (seed=42), not committed to the repo

What changed

File	Purpose
mkdocs.yml	MkDocs Material config with 5-page sidebar nav
docs/index.md	Tutorial: data creation, analysis, plot, summary, report, raw inferences, covariates, parameters, validity
docs/api.md	API reference for CausalImpact, ModelOptions, CausalImpactResults
docs/images/.gitkeep	Placeholder for CI-generated plot images
scripts/generate_docs_assets.py	Generates causal_impact_plot.png with fixed seed (headless Agg backend)
tests/test_docs_assets.py	9 tests: 5 normal, 3 boundary, 1 error case
.github/workflows/docs.yml	CI: Rust build → uv sync → generate assets → mkdocs build --strict → deploy
pyproject.toml	Add [docs] optional-dependencies (mkdocs-material, pymdown-extensions)
.gitignore	Exclude site/ and docs/images/*.png
uv.lock	Lock file updated with docs dependencies

Architecture

Push to main → docs.yml workflow
  ├── Rust build (cargo via maturin)
  ├── uv sync --all-extras
  ├── python scripts/generate_docs_assets.py → docs/images/causal_impact_plot.png
  ├── mkdocs build --strict (catches broken links)
  └── mkdocs gh-deploy --force → gh-pages branch

Test plan

• pytest tests/test_docs_assets.py -v — 9/9 passed
• ruff check — all checks passed
• mkdocs build --strict — builds without warnings or errors
• After merge: verify https://yuminosukesato.github.io/bsts-causalimpact/ renders correctly

Notes for reviewers

• docs/index.md embeds actual ci.summary() and ci.report() output captured from a real run (seed=42, niter=1000). These are static code blocks, not dynamically generated.
• Existing docs (migration-from-r.md, migration-from-tfp.md, compatibility-matrix.md) are included in the nav as-is with no modifications.
• The workflow only deploys on push to main. PR builds run mkdocs build --strict as a validation step without deploying.