FE: Create page groups

[Jondyr]

Implement this as a button on bottom of the list.
A page should be added automatically to have a valid configuration. This should be implemented on the backend.
Issue for endpoint is implemented here: #15029

• Implement button - "Legg til ny sidegruppe"
• When backend endpoint is ready - connect to the endpoint and make a post call.

Why

As a user, I want to add task groups to better structure tasks into groups of pages. This helps organize the content and improves the navigation experience for the end user, making it easier to understand and follow.

The button to add new groups should be clearly visible and placed outside existing groups, so it’s easy to understand that it adds a new group — not a group inside another group.

Design