Add VitePress user documentation

[drown0315]

Summary

• move internal PRDs, ADRs, calibration notes, and design docs to docs-internal so docs can be the public documentation site
• add a VitePress docs site with guide and reference pages for getting started, Scenario authoring, running Scenarios, CLI usage, and Scenario DSL rules
• add Scenario DSL documentation checks that parse documented YAML examples with the real parser
• add docs-specific GitHub Actions for PR checks and GitHub Pages deployment from docs/.vitepress/dist
• configure VitePress to use /flutter_pilot/ as the GitHub Pages base path while keeping local dev at /

Verification

• dart analyze
• dart test
• dart analyze --fatal-infos --fatal-warnings
• dart test test/scenario_dsl_docs_test.dart test/scenario_dsl_docs_tool_test.dart
• dart run tool/check_scenario_dsl_docs.dart
• npm run docs:build
• GITHUB_PAGES=true npm run docs:build

Closes #106