docs: add 6 verified examples and enable image tracking

- Add docs/examples.md with end-to-end examples covering:
  1. R CausalImpact vignette reproduction (with R vs Python comparison table)
  2. No covariates (local level only, stationary data)
  3. Seasonal data (nseasons=52, weekly cycle)
  4. Dynamic regression (time-varying coefficients)
  5. No effect negative control (correct non-significance detection)
  6. Multiple covariates with spike-and-slab variable selection
- Add plot images for all 6 examples + index.md hero image
- Remove docs/images/*.png from .gitignore (required for GitHub Pages)
- Add Examples nav entry to mkdocs.yml

All examples use synthetic data with known true effects.
Summary table confirms all scenarios produce correct results.

Author: YuminosukeSato

.gitignore

@@ -12,4 +12,3 @@ z-ai/
 .claude/
 *.DS_Store
 site/
-docs/images/*.png