Update header and menu on documentation pages

### Description

When clicking a header item, only the left menu of the corresponding theme should be visible to make navigation easier. See example: https://www.designsystemet.no/grunnleggende

### Additional Information

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### Tasks

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### Acceptance Criterias

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Start with the documentation page linked in the issue and inspect how its header items and theme-specific left menus are assembled. The work is done when selecting a header item shows only the corresponding theme's left menu and navigation is easier to follow.

Written by the indexing model from the issue text.