Update TreeGrid article to benefit from flat hierarchy support

[vursen]

1. Add links to "Columns", "Renderers", "Selection" pages since they also apply to TreeGrid

2. Move the TreeGrid drag&drop example from the Grid article to the TreeGrid article
3. Update the TreeGrid drag&drop example to use flattened hierarchy format
4. Add individual Flow example demonstrating how to implement scrollToItem with flattened hierarchy format

[vursen]

WIP branch: https://github.com/vaadin/docs/tree/tree-grid-article-update