smem: add page (#8163)

pages/linux/smem.md

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+# smem
+
+> Print memory usage for programs.
+> More information: <https://manned.org/smem>.
+
+- Print memory usage for current processes:
+
+`smem`
+
+- Print memory usage for current processes for a every user on a system:
+
+`smem --users`
+
+- Print memory usage for current processes for a specified user:
+
+`smem --userfilter {{username}}`
+
+- Print system memory information:
+
+`smem --system`