Remove ITK prefix from Group (Subgroup) in documentation files

In markdown files, "Group (Subgroup)" subsection should look like:
ImageProcessing (ITK ImageProcessing)

instead of:
ITKImageProcessing (ITKImageProcessing)

Search the repository's markdown files for the "ITKImageProcessing (ITKImageProcessing)" subsection form and review the surrounding documentation. Update each matching Group (Subgroup) entry to use "ImageProcessing (ITK ImageProcessing)"; done means the old prefixed form no longer appears in the relevant markdown files.

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