add ai-classification notebook (#207)

Create contrib folder and add a first jupyter notebook

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Co-authored-by: Aman Mangal <[email protected]>
Co-authored-by: Joshua Goldstein <[email protected]>

Author: 3 people

examples/notebook/README.md

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+A collection of jupyter notebooks that you can run in Jupyter Notebook service  [Colab](https://colab.google/), or locally.
+- [dgraph-ai-classification.ipnyb](dgraph-ai-classification.ipynb)
+
+  This notebook is a companion to the blog Boost your data with LLMs and OpenAI embeddings
+  
+  It lets you experiment yourself with Dgraph and OpenAI API.
+