docs: update tree grid articles and use flat hierarchy in examples

[ugur-vaadin]

Updates Tree Grid articles and examples based on the HierarchyFormat changes.

NOTE: Targets the branch for the scrollToItem section PR.

Closes #4529

[github-actions]

AI Language Review

• PopoverPositioning.java: No issues detected.

• TreeGridScrollToItem.java: The change from new TreeDataProvider<>(treeData) to new TreeDataProvider<>(treeData, HierarchicalDataProvider.HierarchyFormat.FLATTENED) is accurate and improves performance by maintaining the scroll position. No issues with this modification.

• TreeGridDragDrop.java: The addition of the TreeGridDragDrop class, including the setup for drag-and-drop functionality and use of HierarchicalDataProvider.HierarchyFormat.FLATTENED, seems correctly implemented. No issues detected.

• tree-grid-drag-drop.ts: The changes appear to improve the drag-and-drop logic in the example code. The modifications encourage better usage practices by avoiding grid.clearCache(), although this is marked not recommended. No issues found.

Overall, the changes improve functionality and performance, addressing specific use cases like drag-and-drop operations in hierarchical data components.

[vursen]

Let's add the scrollToItem documentation in a separate PR that doesn't introduce the flattened hierarchy yet.

[ugur-vaadin]

Let's add the scrollToItem documentation in a separate PR that doesn't introduce the flattened hierarchy yet.

Extracted scrollToItem section to another PR.