Add the graph refuter to the list of refutations in top-level documentation

[emrekiciman]

Add graph refutation to the list of available refutation methods and the refuter notebook:

• https://py-why.github.io/dowhy/v0.8/user_guide/effect_inference/refute.html
• https://github.com/py-why/dowhy/blob/main/docs/source/example_notebooks/dowhy_refuter_notebook.ipynb

Here's a notebook that demonstrates the graph refutation: https://py-why.github.io/dowhy/v0.8/example_notebooks/graph_conditional_independence_refuter.html