Add gradient operator docs

Author: nHackel

docs/src/tutorials/gradient.jl

@@ -0,0 +1,10 @@
+# # Gradient Operator
+include("../../util.jl") #hide
+# This operator computes a direction gradient along one or more dimensions of an array:
+gop = GradientOp(eltype(image); shape = (N, N), dims = 1)
+gradients = reshape(gop * vec(image), :, N)
+fig = Figure()
+plot_image(fig[1,1], image, title = "Image")
+plot_image(fig[1,2], gradients[:, :], title = "Gradient", colormap = :vik)
+resize_to_layout!(fig)
+fig