refactor: configure page menu to show 2nd level headings (#4854)

Author: vursen

articles/components/tree-grid/data-providers.adoc

@@ -5,6 +5,7 @@ order: 30
 ---
 
 = Data Providers (Flow)
+:toclevels: 2
 
 Tree Grid supports connecting to different types of data sources using hierarchical data providers. For simple use cases, it offers a built-in [classname]`TreeData` structure along with a ready-made [classname]`TreeDataProvider`, which together provide an easy way to create and manage hierarchies in memory. It also supports custom hierarchical data providers to fetch data from other sources, e.g. a database.