groupadd, groupmod, groupdel: add page (#1194)

pages/linux/groupadd.md

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+# groupadd
+
+> Add user groups to the system.
+
+- Create a new Linux group:
+
+`groupadd {{group_name}}`
+
+- Create new group with a specific groupid:
+
+`groupadd {{group_name}} -g {{group_id}}`

pages/linux/groupdel.md

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+# groupdel
+
+> Delete existing user groups from the system.
+
+- Delete an existing group:
+
+`groupdel {{group_name}}`

pages/linux/groupmod.md

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+# groupmod
+
+> Modify existing user groups in the system.
+
+- Change the group name:
+
+`groupmod -n {{new_group_name}} {{old_group_name}}`
+
+- Change the group id:
+
+`groupmod -g {{new_group_id}} {{old_group_name}}`