diffimg: add page (#14105)

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/linux/diffimg.md

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+# diffimg
+
+> Calculate intersection between two images.
+> Note: the supported extensions are `.png`, `.gif`, `.jpg`, `.ps`.
+> More information: <https://manned.org/diffimg>.
+
+- Calculate the intersection between images and output an image where each pixel is the difference between corresponding pixels in input images:
+
+`diffimg {{path/to/input_image1.ext}} {{path/to/input_image2.ext}} {{path/to/output_image.ext}}`