gifdiff: add page (#8404)

pages/common/gifdiff.md

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+# gifdiff
+
+> Compare two GIFs for identical visual appearance.
+> See also: `gifsicle`.
+> More information: <https://www.lcdf.org/gifsicle>.
+
+- Check how GIFs differ:
+
+`gifdiff {{path/to/first.gif}} {{path/to/second.gif}}`
+
+- Check if GIFs differ:
+
+`gifdiff --brief {{path/to/first.gif}} {{path/to/second.gif}}`