cmatrix: add page (#4091)

pages/common/cmatrix.md

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+# cmatrix
+
+> Shows a scrolling Matrix like screen in the terminal.
+> More information: <https://github.com/abishekvashok/cmatrix>.
+
+- Enable asynchronous scrolling:
+
+`cmatrix -a`
+
+- Display red text:
+
+`cmatrix -C {{red}}`
+
+- Enable rainbow mode:
+
+`cmatrix -r`
+
+- Set screen update delay to 2 centiseconds:
+
+`cmatrix -u {{2}}`