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.

首先查看本 issue 中描述的资源加载方式和 RenderAssetUsage API，然后阅读 issue #12057 中的相关背景。完成的标准是：示例按顺序加载资源，使用 RenderAssetUsage 将每个资源移到 GPU，并解释为什么这会减少 Web 平台上未回收的内存。

由索引模型根据 Issue 内容生成。