How to print at custome scales

### Descriptio
Add to the dev guide some documentation on how custom scales can be set for the print plugin (localConfig.json), sample below:

```
{
  "name": "Print",
  "cfg": {
    "useFixedScales": true, // ?
    "mapPreviewOptions": {
        "enableScalebox": true, // ?
        "enableRefresh": true // ?
    }
  }
}
```
 
### Other useful information (optional):
