Refactor code to use standalone widgets instead of helper methods

[khanhnwin]

Page URL

https://docs.flutter.dev/cookbook/effects/shimmer-loading

Page source

No response

Describe the problem

Some of our docs have example code which uses helper methods instead of using standalone widgets. It's a general recommendation that we break out help methods into their own standalone widgets.

• Shimmer loading #6810
• Staggered menu item #6809

Expected fix

We should refactor the code and break out all of the helper methods into standalone widgets.

Additional context

No response