Option to disable/modify auto-generated page numbers on NavigationBar Components

### Description

Currently, there is an automatic numbering system applied to items on a [NavigationBar component](https://docs.altinn.studio//app/development/ux/pages/navigation/#navigation-bar). While this can be useful in some scenarios, there are instances where these auto-generated numbers are either not needed or need to be customized - eg. when hiding/showing pages based on user input, the auto generation can in some cases result in a mismatch between page number and the number generated in the navigation bar.

### Additional Information

**Suggestion:** Add a property for turning numbering on/off on the component itself, something like
```JSON
{
"id": "navbar-page-1",
"type": "NavigationBar",
"showPageNumbers": false
}
```

Start with the NavigationBar component and the linked navigation documentation. Trace how page numbers are generated, then determine how the proposed configuration should control numbering when pages are hidden or shown. Done means the component supports the agreed numbering behavior without mismatches.

Written by the indexing model from the issue text.