Seems all syntax requires the admin manually type a "root" category.
{{#categorytree:Foo}}
But my wiki doesn't have a "root" category and subcategories-- it just has a collection of categories.
How to show all categories, without typing a "root" category?
Might be here, unsure: Extension:CategoryTree#Using MediaWiki: namespace pages
This returns nothing:
{{#categorytree:{{root category}}}}