Add notebook with NYC MTA example

[melissawm]

A few questions remain:

• Do we want to use .ipynb files or .md files? Using markdown files for notebooks is possible with Jupytext, but it would add an extra step for users to be able to open the file.
• Independent of the file format, do we want to keep the outputs in the example file? Since this is a static example we may want to display in the documentation, I've left the outputs for now, but happy to clean up if that's preferred.

Any feedback is appreciated.