Use the shorthand functions to construct Vals and UiRects in the UI examples

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

Consistency, teaching best practices.

## What solution would you like?

Always use the shorthand syntax in UI examples. E.g.

- `Val::Px` -> `px()`
- `Val::Percent` -> `percent()`

## What alternative(s) have you considered?

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## Additional context

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