Add example/simplify usage of multi target viewports to render on top of other UI elements

[nicopap]

Since camera-driven rendering, it should be possible to render the log UI to a different target than the default one and assign it a priority higher than it, allowing to draw over everything esle.

Alternatives (choice still open)

• Add an example demonstrating how to do this in user code
• Add a config toggle that let the library do it for the user
• See where Add support for multi viewports and windows to UI bevyengine/bevy#5892 goes, this should 100% be the favored solution.