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.

Start by reviewing the asset-loading approach described in this issue and the RenderAssetUsage API, then read the related context in issue #12057. Done means an example loads assets sequentially, moves each onto the GPU with RenderAssetUsage, and explains why this reduces unreclaimed memory on web platforms.

Written by the indexing model from the issue text.