watch: add page

pages/common/watch.md

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+# watch
+
+> Execute a program periodically, showing output fullscreen.
+
+- Repeatedly run a command and show the result:
+
+`watch {{command}}`
+
+- Re-run a command every 60 seconds:
+
+`watch -n {{60}} {{command}}`
+
+- Monitor the contents of a directory, highlighting changes as they appear:
+
+`watch -d {{ls -l}}`