Set up documentation with MkDocs and Material theme (#6)

Add MkDocs and related dependencies (mkdocs-material, mkdocstrings,
pymdown-extensions) to dev dependencies. Update CI workflow to use
Material theme for documentation deployment.

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -42,6 +42,10 @@ dev = [
   "pytest-mock >= 3.12.0",
   "pytest-cov >= 5.0.0",
   "pandas-stubs >= 2.2.3.250527",
+  "mkdocs >= 1.6.1",
+  "mkdocs-material >= 9.7.6",
+  "mkdocstrings[python] >= 1.0.4",
+  "pymdown-extensions >= 10.21.2",
 ]
 
 [tool.uv.build-backend]