Move Sphinx toctree directives from index.rst to index.md to prevent root document name collisions during ReadTheDocs builds. Update all documentation guides and API references to feature TabularConfig and TabularMechanism as the primary in-memory synthesis interface, and surface DiscreteConfig and DiscreteMechanism at the top-level dpsynth API. Add a complete, self-contained standalone example demonstrating domain specification, config setup, mechanism calibration, data loading, synthesis, and inspection. - #153