Add the Internal Category <pre>[[Category:Internal]]</pre> and protect it using ACL.
Create a category by searching for <nowiki>Category:Name</nowiki>, or by clicking a link to the category. Add a page to a category by including the tag <nowiki>[[Category:Name]]</nowiki>
To create a link to a category, use a leading colon before the category name (without this colon, the current page would be added to the category):
<nowiki>[[:Category:Help]] -> Category:Help</nowiki>
To change the link text, write the text inside the link tag after a pipe:
<nowiki>[[:Category:Help|Help category]] -> Help category</nowiki>
See https://www.mediawiki.org/wiki/Help:Categories for more information.