shred: add page

pages/common/shred.md

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+# shred
+
+> Overwrite files to securely delete data
+
+- Overwrite a file:
+
+`shred {{file}}`
+
+- Overwrite a file, leaving zeroes instead of random data:
+
+`shred --zero {{file}}`
+
+- Overwrite a file 25 times:
+
+`shred -n25 {{file}}`
+
+- Overwrite a file and remove it:
+
+`shred --remove {{file}}`