zdiff: add page (#8636)

pages/common/zdiff.md

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+# zdiff
+
+> Invoke `diff` on gzipped files.
+> More information: <https://manned.org/zdiff>.
+
+- Compare two files, uncompressing them if necessary:
+
+`zdiff {{path/to/file1.gz}} {{path/to/file2.gz}}`
+
+- Compare a file to a gzipped archive with the same name:
+
+`zdiff {{path/to/file}}`