Add getting started notebook (#148)

As a follow-up to #140, this adds/updates some additional tutorial
notebooks

- [Make Counterfactual
Graph](https://github.com/y0-causal-inference/y0/blob/getting-started-notebook/notebooks/Make%20Counterfactual%20Graph.ipynb)
- [Getting
Started](https://github.com/y0-causal-inference/y0/blob/getting-started-notebook/notebooks/Getting-Started.ipynb)

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Co-authored-by: njmmerrill <[email protected]>
Co-authored-by: Jeremy Zucker <[email protected]>

Author: 3 people