Add `itk.DiscreteGaussianImageFilter` Python example

[https://examples.itk.org/src/filtering/smoothing/smoothimagewithdiscretegaussianfilter/documentation](https://examples.itk.org/src/filtering/smoothing/smoothimagewithdiscretegaussianfilter/documentation)

Start with the linked example documentation page and inspect how the existing DiscreteGaussianImageFilter example is represented in this repository. Identify the Python example and its documentation entry points, then add the missing Python example so the filter is covered alongside the existing material.

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