Add docs for the drag and drop feature of the TreeView (#168)

* feat(treeview): drag and drop docs

* chore(treeview): remove redundant slug

* chore(treeview): update metadata

* chore(treeview): minor improvements on drag-and-drop article wording

* chore(treeview): explain when the OnDrop fires when drag and drop between different trees

* chore(treeview): remove the button that toggles draggable for the flat data example

* chore(treeview): add explanation to the DropPosition enum

Co-authored-by: Marin Bratanov <m.bratanov@gmail.com>

Author: svdimitr