Update or Remove "ICESat-2’s Nested Variables" tutorial

[asteiker]

https://icepyx.readthedocs.io/en/latest/example_notebooks/IS2_data_variables.html

This tutorial may need to be removed entirely given that it is focused on variable discovery.

[JessicaS11]

At the very least, this tutorial will remain because variable discovery and management works on local files. Further, it is critical for the read module in that capacity. So it will require updates, but not removal.