Feathers needs better examples

[alice-i-cecile]

There are multiple steps to this, each of which probably deserves its own PR for ease of review:

• Rename the existing example to feathers_example_showcase.
• Move all of the feathers examples into a subfolder under UI examples.
• Create a new, minimal getting-started-with-feathers example.
• Create a new very short example for each feathers widget.

[viridia]

I would prefer to wait on this until after the bsn migration, as it will both make the construction of examples easier, and avoid promulgating ugly code that we really don't want users to learn from.