WebGPU Compilation Steps

Hi all,

I think it would be helpful to have a page somewhere in the docs that explains how to compile a bevy app with either (or both) the WebGL2 + WebGPU backends.

After some discussion with people on Discord, I was able to get up-and-running, but the process seems a bit awkward and difficult for new users to figure out on their own. For reference, here are the steps I followed. Maybe this is not the "best" way, but it seems to work OK for my purposes.

First, I added the following to my Cargo.toml file:

```
[dependencies.bevy]
version = "0.11.1"
default-features = false # This bit is important
features = [
"animation",
"bevy_asset",
"bevy_audio",
# ... the rest of the bevy features you need EXCEPT webgl2
]

[features]
webgl2 = ["bevy/webgl2"]
```

I am then able to build for either WebGL2 or WebGPU.

For WebGL2:
```
cargo build --features webgl2 --target wasm32-unknown-unknown
```

For WebGPU:
```
cargo build --target wasm32-unknown-unknown
```

Additionally, when targeting WebGPU, you must have the following environment variable set:
```
RUSTFLAGS=--cfg=web_sys_unstable_apis
```

From here, you can run `wasm-bindgen` on the resulting .wasm file and load it on a webpage as usual.

Perhaps there is a simpler way to convey this information, but I just wanted to get the discussion started and help out anyone else who might be struggling with the same thing.

Start with the Cargo.toml feature example and the WebGL2/WebGPU cargo build commands described in the issue, then check how the resulting .wasm file is passed to wasm-bindgen. Document the setup, the RUSTFLAGS requirement for WebGPU, and the commands for both backends; done means a new user can follow the page to build either target.

Written by the indexing model from the issue text.