docs: finish tutorials documentation

[chaewon-research]

Fixes #21

What changed

• Removed unfinished authoring text (Create examples/basic_usage.rst:) and the broken diagram reference (basic_flow.png).
• Made the basic usage example self-contained with explicit imports and synthetic data generation.
• Added a tutorials index linking directly to all 7 repository notebooks.
• Cleared the stale ModuleNotFoundError output from geometric_knn_causal_discovery_example.ipynb.

Validation

• Built the Sphinx documentation locally with zero tutorial-related warnings.
• Verified the Tutorials page renders cleanly in the browser with functional links and syntax highlighting.
• Ran the full default pytest suite (381 passed, 17 deselected).
• Audited the git diff (git diff --check clean, 2 files changed, +38 / -118).