docs: def _build_category_tree(slug, reference=None, items=None):
      """
      Builds a recursive tree with category relations as children.
      """
  
      if items is None:
          items = []
  
      for key in reference:
          category = reference[key]
  
          if category["parent"] == slug:
              children = _build_category_tree(category["nicename"],
                                              reference=reference)
              category["children"] = children
              items.append(category)
  
      return items