Treegrid in 18.2

Hello,

Since 18.2, `FlatTreeControl`, `MatTreeFlatDataSource` and `MatTreeFlattener` are deprecated, and I was relying on them to create a treegrid, as suggested by @andrewseguin in https://github.com/angular/components/issues/15187#issuecomment-463715842:

> You should be able to do this using the table currently, and to make it easier you can use the `TreeControl` to help you traverse your data and manage expansion state
> https://stackblitz.com/edit/angular-m77g7e-semvxp?file=app%2Ftable-basic-example.html

So I was wondering if there was a new recommended way of creating a treegrid, or if I needed to reimplement those deprecated classes. 🤔

Thanks for your help!

Read the deprecation context for FlatTreeControl, MatTreeFlatDataSource, and MatTreeFlattener, along with the treegrid example linked from issue #15187. Confirm whether Angular Material has a supported replacement or document the recommended treegrid path; done means the supported approach is clear.

Written by the indexing model from the issue text.