Zonal Avg Notebook Update (#1403)

* o Add another example to zonal averaging, also wrap the isel logic in a try/except so that if a caller passes a dimension name that doesn’t exist then raise ValueError; test case for the same

* o Fix missed datasetpath

* o Add comment about crossection notebook and some more fixes

* Clarify NE30 zonal mean narrative

* Clean up default latitude list

* o Fix comments and text

* o Update documentation - Zonal notebook.

* o Update documentation - Zonal notebook.

* Fix broken URL

* A few small rephrasing

---------

Co-authored-by: Orhan Eroglu <32553057+erogluorhan@users.noreply.github.com>
Co-authored-by: erogluorhan <erogluorhan@gmail.com>

Author: rajeeja