nmon.md: add page (#1826)

pages/linux/nmon.md

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+# nmon
+
+> A system administrator, tuner, and benchmark tool.
+
+- Start nmon:
+
+`nmon`
+
+- Save records to file ("-s 300 -c 288" by default):
+
+`nmon -f`
+
+- Save records to file with a total of 240 measurements, by taking 30 seconds between each measurement:
+
+`nmon -f -s {{30}} -c {{240}}`