Add nx-cugraph introduction notebook to repo (#4677)

## Proposed Changes

This PR adds an introduction notebook to the `notebooks/demo` directory
of the repository. Click the link to view the files directory in the dev
branch.
* [The introduction to `nx-cugraph`
notebook](https://github.com/rapidsai/cugraph/blob/1a70b7ad588bcc1dc6f65af8fe07149ed8083ef0/notebooks/demo/accelerating_networkx.ipynb)

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Co-authored-by: Erik Welch <[email protected]>