Document how dark mode works in Designsystemet

[mrosvik]

We need clear documentation that explains how dark mode works in Designsystemet. This should help both designers and developers understand the underlying principles, how it is implemented, and how to use it correctly in their solutions.

The documentation should cover both design and technical perspectives, and explain any limitations or important considerations.

• There is a dedicated documentation page for dark mode in Designsystemet
• The page explains the overall concept and purpose of dark mode
• It describes how colours and tokens behave in dark mode
• It explains how to activate and use dark mode in practice
• Accessibility considerations are clearly described
• The content is understandable for both designers and developers

[Barsnes]

I think we should write about light mode as well