Add cubeside example.

examples/misc/cube_sides.md

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+# Plot on the sides of a cube
+
+This example shows how we can plot images on the sides of a cube. We can print them either on the back side
+and use the left available space to plot another *thing*, as in:
+
+\begin{examplefig}{}
+```julia
+using GMT
+
+viz(GMT.peaks(N=100), zsize=8, facades=(GMT.TESTSDIR * "cenora_base.jpg",
+ GMT.TESTSDIR * "bunny_cenora.webp",
+ GMT.TESTSDIR * "burro_cenora.webp"))
+```
+\end{examplefig}
+
+The above example used the helper function \myreflink{imshow} (well, it's alias ``viz``) that knew what to do when
+it found the keyword `facades`, but in fact the work was done by the function \myreflink{cubeplot}. Next example shows
+how to call that function directly to plot three images in the front sides of the cube.
+
+\begin{examplefig}{}
+```julia
+using GMT # Hide
+
+cubeplot(GMT.TESTSDIR * "seila_gray.jpg",
+ GMT.TESTSDIR * "seis_section_rgb.jpg",
+ GMT.TESTSDIR * "seis_section_gray.jpg",
+ zsize=6, show=true)
+```
+\end{examplefig}
+