Add In-World UI example

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

When #5476 is solved, we need example(s) showcasing how to setup and use UI in-world.

## What solution would you like?

Examples showcasing the following features:

- How to display UI in the world
- Display multiple separate UI elements in the world
- How to interact with in-world UI

These can be one and the same example, or multiple depending on how big they need to be in order to showcase what they need. Just showing a couple of different UI elements that isn't 100% camera-facing with buttons that can be pressed should be enough to showcase what we need, but feel free to come up with something more interesting.

Review the resolution of issue #5476 before starting, then inspect existing Bevy examples to determine where an in-world UI example belongs. Done means an example demonstrates displaying multiple separate UI elements in the world and interacting with them, including buttons that can be pressed.

Written by the indexing model from the issue text.