function: add page (#15241)

pages/common/function.md

@@ -0,0 +1,12 @@
+# function
+
+> Define a function.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#Shell-Functions>.
+
+- Define a function with the specified name:
+
+`function {{func_name}} { {{echo "Function contents here"}}; }`
+
+- Run a function named `func_name`:
+
+`func_name`