Add nx-cugraph notebook for showing accelerated networkX APIs (#3830)

This PR adds a notebook showing how to use the `nx-cugraph` package to accelerate your NetworkX workflow.

closes rapidsai/graph_dl#277

Authors:
  - Dylan Chima-Sanchez (https://github.com/betochimas)
  - Rick Ratzel (https://github.com/rlratzel)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)
  - Erik Welch (https://github.com/eriknw)

URL: #3830