Add a raster reprojection example

[edsaac]

Rationale

Plotting a raster image with ax.imshow requires data to be warped and reprojected. This does not works well if the map's extent is not set in advance. I am adding an additional example to the documentation to cover that common pitfall:

• imshow fails when transform projection is different that the axes projection #2480
• Inconsistent imshow interpolation #1468

Implications

This is just an additional example to put in the documentation; no functionality in the package is modified. Below is a screenshot of rebuilding the docs with sphinx:

sphinx-build -M html docs/source/ docs/build

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