duff: add page (#23268)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Wiktor Perskawiec <wiktor@perskawiec.cc>

pages/linux/duff.md

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+# duff
+
+> Duplicate file finder.
+> More information: <https://github.com/elmindreda/duff>.
+
+- Find duplicate files recursively:
+
+`duff -r {{path/to/directory}}`
+
+- List all but one file from each cluster of duplicates:
+
+`duff -er {{path/to/directory}}`
+
+- List unique files:
+
+`duff -ur {{path/to/directory}}`
+
+- Use SHA-256 instead of SHA-1:
+
+`duff -d sha256 {{path/to/directory}}`
+
+- Compare files byte by byte instead of using digests:
+
+`duff -t {{path/to/directory}}`
+
+- Include hidden files and directories when searching recursively:
+
+`duff -ar {{path/to/directory}}`