UI `ImageNode` examples

## What problem does this solve or what need does it fill?

This is probably the most important UI widget component alongside Text, yet we have around twenty text examples and not a single basic "this how you lay out an image example. This is despite the many users who struggle to lay out images while preserving their aspect ratio.

There also seem to be some bugs with measure funcs where an incorrect amount of vertical space is allocated. `testbed_ui` should have image scenes with all the edge cases. At the moment it just has a single image scene that displays the bevy logo without any constraints.

## What solution would you like?

* An `image_node` example that goes through the common ways to use `ImageNode`.
* A more advanced example that goes deeper into how automatic and manual image sizing work.
* More `testbed_ui` `image` scenes. Maybe one showing some common image layouts, and a more advanced scene demonstrating the bugs and edges.

Start with the existing single image scene in testbed_ui and the ImageNode usage in the examples. Add an image_node example covering common layouts, a more advanced example for automatic and manual sizing, and testbed_ui image scenes for layout and sizing edge cases; done means the examples cover the requested cases and expose incorrect vertical space allocation.

Written by the indexing model from the issue text.