Help:Categories and namespaces
The FHB wiki uses the additional namespace Recipe for grouping, well, recipes. All you have to do when creating a new one, is to prefix the page title with "Recipe:", like Recipe:Khlav_Kalash. It will be listed in the recipe index automatically. Further subgrouping via categories is planned, but not implemented yet.
In the main namespace, without prefix, categories are currently used for
To mark a page to belong to one or more categories, just add a tag at the bottom when editing:
Have a look at the existing pages for examples.