Simplify Cross-slope_section.ipynb example

[navidcy]

Simplify https://github.com/COSIMA/cosima-recipes/blob/main/DocumentedExamples/Cross-slope_section.ipynb into two examples, one for a transect and one for an along/cross slope decomposition.

Originally posted by @ongqingyee in #332 (comment)

[adele-morrison]

This was closed by #420