Use consistant spawning patterns in the UI examples

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

The UI examples currently use many different patterns to spawn the UI hierarchies:

- `.with_children`
- `children!`
- `Children::spawn`

And when creating reusable components:

- `Commands` and `spawn`
- `-> impl Bundle`

When trying to learn idiomatic Bevy UI usage, this is confusing. I want to know which pattern to use in which situation.

## What solution would you like?

We should have one preferred way to do things and use that pattern consistently in our examples to teach it.

All UI examples should be updated to use a consistent spawning pattern.

## What alternative(s) have you considered?

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## Additional context

It's likely a good idea to wait for this until BSN lands, as it will add yet another pattern (which will likely be the preferred one).