Add comprehensive documentation for the graph-based pipeline DSL:
- docs/graph-dsl.md: Complete DSL reference with TOML, Python, and CLI syntax
- docs/GRAPH_SYSTEM_SUMMARY.md: Implementation overview and architecture
- examples/graph_dsl_demo.py: Demonstrates imperative Python API usage
- examples/test_graph_integration.py: Integration test verifying pipeline execution
The documentation follows a wiki-like approach with navigable structure:
- Overview section explaining the concept
- Syntax examples for each format (TOML, Python, CLI)
- Node type reference table
- Advanced features section
- Comparison with old XYZStage approach
This provides users with multiple entry points to understand and use the
new graph-based pipeline system.
