[Decoupling] Proposal: Decouple cupertino_ui's dartDoc imports of material_ui

Cupertino contains a few dartDoc imports of Material. For example, CupertinoMenuAnchor [imports](https://github.com/flutter/flutter/blob/764a079e567bba1ab0d93b68e4d30bb1181fc1bf/packages/flutter/lib/src/cupertino/menu_anchor.dart#L5) Material in order to [use a docs macro](https://github.com/flutter/flutter/blob/764a079e567bba1ab0d93b68e4d30bb1181fc1bf/packages/flutter/lib/src/cupertino/menu_anchor.dart#L1785). I'm unsure if all dartDoc imports are for similar reasons, but I imagine Cupertino's docs might also directly refer to Material classes in some places.

This causes cupertino_ui to need material_ui as a devDependency, which would be nice to remove.

## TODOs

* Audit dartDoc imports of material_ui from cupertino_ui.
* Decide if there are any that make sense as-is, or if we can remove them all.
* Either remove them or close this issue.