Document embedding carts using a wasm4.org iframe
 ```html
<iframe src="https://wasm4.org/embed/#?url=https://yoursite.com/yourcart.wasm" allow="fullscreen; gamepad; autoplay" frameborder="0"></iframe>
```

We should document this in the [Distribution guide](https://wasm4.org/docs/guides/distribution) and maybe add an "Embed" button for each game on the Play section.

We can also add extra params to the iframe URL for common settings (mute audio, disable menu, etc).

_Originally posted by @aduros in https://github.com/aduros/wasm4/issues/615#issuecomment-1454807068_
           