docs-bug(CdkTree): Most examples are broken

[ManelBH]

Documentation Feedback

The following examples in the cdk tree page are broken or incomplete and won't compile in Stackblitz:

cdk-tree-nested-level-accessor-example
cdk-tree-nested-children-accessor-example
cdk-tree-flat-level-accessor-example
cdk-tree-flat-children-accessor-example
cdk-tree-complex-example

Affected documentation page

https://material.angular.io/cdk/tree/examples

[pedroestabruxelles]

It was reported by me here: #30159

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