duf: add page (#8171)

pages/common/duf.md

@@ -0,0 +1,20 @@
+# duf
+
+> Disk Usage/Free Utility.
+> More information: <https://github.com/muesli/duf>.
+
+- List accessible devices:
+
+`duf`
+
+- List everything (such as pseudo, duplicate or inaccessible file systems):
+
+`duf --all`
+
+- Only show specified devices or mount points:
+
+`duf {{path/to/directory1 path/to/directory2 ...}}`
+
+- Sort the output by a specified criteria:
+
+`duf --sort {{size|used|avail|usage}}`