Fix up plotting notebooks

[dcherian]

• move data creation to xarray.tutorial.load_dataset("temperature_gradients")?
• Fix exercise formatting
• More text in hvplot notebook

[scottyhq]

Also noticing that there is currently a mix of plt.rcParams and matplotlib retina settings. If retina is not set, plots appear stretched on modern monitors:

https://tutorial.xarray.dev/intermediate/01-high-level-computation-patterns.html#load-example-dataset