[cupertino] Update `CupertinoButton` & `CupertinoNavigationBar` documentation

[TahaTesser]

More details in https://github.com/flutter/flutter/pull/91602#pullrequestreview-777815883

A new CupertinoButton constructor for this special case may not be warranted since it's only setting the padding and minSize defaults. The docs already call out that the padding will need to be different in a CupertinoNavigationBar and it's not clear the extra constructor adds much over documentation.

Instead, we could extend the documentation on CupetinoButton.padding and CupertinoNavigationBar.leading to include a doc snippet to make it more likely that people will see it in the right context.

• [ ] Update CupertinoButton.padding & CupertinoNavigationBar.leading
• [ ] Add code snippets that show case CupertinoButton's correct usage in the CupertinoNavigationBar
• [ ] Add Leading/Trailing examples for CupertinoNavigationBar with Align Widget

[flutter-triage-bot[bot]]

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