sh: add page

pages/common/sh.md

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+# sh
+
+> Bourne shell.
+> The standard command language interpreter.
+
+- Start interactive shell:
+
+`sh`
+
+- Execute a command:
+
+`sh -c {{command}}`
+
+- Run commands from a file:
+
+`sh {{file.sh}}`
+
+- Run commands from STDIN:
+
+`sh -s`