Document how to use Vite with preload scripts

### Pre-flight checklist

- [X] I have read the [contribution documentation](https://github.com/electron/forge/blob/main/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project uses.
- [X] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem description

The current [Vite Plugin documentation](https://www.electronforge.io/config/plugins/vite) shows how to include a preload script into a Vite build, but it does not explain how the magic globals get named or how to address them in a `new BrowserWindow()` call.

I'm looking into moving from Webpack, so I'm thinking about this from Webpack point of view, but the example doesn't make it clear how to address scripts from the `config.build` array.

### Proposed solution

Expand to the `main.js` example code to show a proper `new BrowserWindow()` call.

Something like:
```JS
const mainWindow = new BrowserWindow({
width: 800,
height: 600,
webPreferences: {
preload: PATH_USING_MAGIC_GLOBAL
}
});
```

### Alternatives considered

n/a

### Additional information

_No response_