useradd, userdel, usermod: move from common to linux directory

pages/linux/useradd.md

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+# useradd
+
+> Create a new user
+
+- Create new user
+
+`useradd {{name}}`
+
+- Create new user with a default home directory
+
+`useradd -m {{name}}`
+
+- Create new user with specified shell
+
+`useradd -s {{/path/to/shell}} {{name}}`
+
+- Create new user with supplementary groups (mind the lack of whitespace)
+
+`useradd -G {{group1,group2}} {{name}}`

pages/linux/userdel.md

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+# userdel
+
+> Remove a user
+
+- Remove a user and their home directory
+
+`userdel -r {{name}}`

pages/linux/usermod.md

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+# usermod
+
+> Modifies a user account
+
+- Change a user's name
+
+`usermod -l {{newname}} {{user}}`
+
+- Add user to supplementary groups (mind the whitespace)
+
+`usermod -a -G {{group1,group2}} {{user}}`
+
+- Create a new home directory for a user and move their files to it
+
+`usermod -m -d {{/path/to/home}} {{user}}`