Manual:$wgNamespaceProtection

This setting allows a wiki to customize default protection levels by namespace. By default, the only restriction is that the MediaWiki namespace can only be edited by users with the  permission (by default: sysops).

The keys of the array are namespace numbers, and the values are simple arrays of permission names. If you list more than one permission for a given namespace, a user must have all of them to edit pages in that namespace.