docs-bug(treeview): Add guide on how to migrate from old treeview to new treeview

### Documentation Feedback

Following the sudden deprecation of the various tree controllers, it would be nice if there were some kind of documentation on how to remove tree controller and migrate to the new way of doing things. Also, the documentation still contains references to `treeControl`, which it shouldn't because the treeControl has been deprecated?


A simple migration guide would be warranted as well because there are tonnes of search results and guides on how to use the tree that just became out of date, so if there was an easy way to adapt code to the new way of doing things, that would be ideal.

### Affected documentation page

https://material.angular.io/components/tree/overview

Start with the tree overview page at https://material.angular.io/components/tree/overview and review its current treeControl references and guidance. Document migration from the deprecated tree controllers to the new tree approach, and remove or update outdated references so the page provides a clear migration path.

Written by the indexing model from the issue text.