When editors come to the Data namespace on Commons, they sometimes want to create a page, and do not understand why it is not possible. We should show a proper explanation text. Since this is Commons-only, there is not much sense to do it from code.

TODO: Modify MediaWiki:Noarticletext-nopermission to check if the current namespace is DATA, and if so, show text that points to .tab and .map help pages, and quickly explains what they do.