newgrp: add page (#2944)

* Create newgrp.md

* Update newgrp.md

* Update newgrp.md

* Update newgrp.md

pages/linux/newgrp.md

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+# newgrp
+
+> Switch primary group membership.
+
+- Change primary group membership:
+
+`newgrp {{group_name}}`
+
+- Reset primary group membership to user's default group in /etc/passwd:
+
+`newgrp`