Add `nx-cugraph` introduction notebook to repo

[nv-rliu]

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

[review-notebook-app[bot]]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[BradReesWork]

/merge