Consider a more practical showcase for `2d_on_ui` example

Alice mentioned a desire for a potential followup here: https://bsky.app/profile/alice-i-cecile.bsky.social/post/3lqnthgrcee2t so here's an issue with my thoughts.

For reference, the example PR: #18513.

# Problem

The example currently shows a sprite rotating in ways that would be hard to achieve using pure UI, so it's demonstrating something potentially useful, but it's not clear to me at least what the practical applications might be.

The [linked discussion](https://github.com/bevyengine/bevy/discussions/11223) listed as motivation for the PR proposes the following question:

> I want to draw a texture, a 3D mesh, a minimap… anything! on top of the bevy UI. How do I do this?

But the answer to most of that in recent/upcoming versions of Bevy is pretty likely to involve `ImageNode`, `ViewportNode`, or `UiMaterial`.

# Solution?

Ideally, we should find a compelling reason to draw on top of UI and rework the example slightly to show that.

We may also want to cross-reference other related examples.

# Also

I think we should consider moving the example to the `ui` subfolder. The example is showing 2d stuff on UI, but the concept isn't really 2d-specific.

Start by locating the `2d_on_ui` example and reading the referenced PR #18513, then review discussion #11223 and the linked motivation. The work is done when the example demonstrates a compelling practical use of drawing over UI, cross-references related examples where useful, and its placement in the `ui` folder has been resolved.

Written by the indexing model from the issue text.