docs(Mat-tree): Mat nested tree node example

### Documentation Feedback

 Can we have a more detailed document/example of how mat nested tree node can traverse to parent upto root and get the full path between leaf node and root?
I believe flat tree has a very good example, but not nested tree. Is it because nested tree cannot do such thing??

### Affected documentation page

https://v15.material.angular.io/components/tree/examples