Add an example demonstrating how to load your assets sequentially to reduce memory usage on web platforms

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

On WASM, memory cannot be reclaimed / freed. As a result, loading all of your assets up front in a loading-screen pattern will balloon your memory usage and it will never go back down. See https://github.com/bevyengine/bevy/issues/12057 for more context.

## What solution would you like?

Add an example demonstrating how to load assets one at a time, and then moving them onto the GPU using `RenderAssetUsage` once loaded.

Explain why this is useful for web.

## What alternative(s) have you considered?

We could make this the default behavior on wasm.

## Additional context

This came up when helping a commercial user debug performance issues on iOS.

Empieza revisando el enfoque de carga de assets descrito en este issue y la API RenderAssetUsage; después, lee el contexto relacionado en el issue #12057. Se considera terminado cuando un ejemplo carga los assets secuencialmente, mueve cada uno a la GPU con RenderAssetUsage y explica por qué esto reduce la memoria no liberada en plataformas web.

Escrito por el modelo de indexación a partir del texto del issue.