Remove absolute positioning from UI examples where possible

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

Absolute positioning is straightforward and easy to understand. However, many users struggle with using the other style properties correctly. Providing more examples of their use could help users better understand these properties.

## What solution would you like?

Instead of using absolute positions, use combinations of the other style properties to achieve the same effect.

Start by locating the Bevy UI examples that use absolute positioning and review how their layouts are constructed. Replace absolute positioning where equivalent style properties can achieve the same result, while preserving the examples' behavior; done means the affected examples demonstrate those properties clearly without unnecessary absolute positions.

Written by the indexing model from the issue text.