docs: Update docs to clarify user expectation on Multitheming

[karlbaumhauer]

Description

As a user of the solid design system,
I want to clearly understand what I can expect from the multitheming when reading the docs,
and I want to understand the difference in regards to a white-label-libray,
so that I can align with my stakeholder on the expected design.

Where to put the documentation

Storybook

DoR

• Item has business value
• Item has been estimated by the team
• Item is clear and well-defined
• Item dependencies have been identified

[dpuetzdw]

Text to Use:
Multitheming
With Multitheming, the Solid Design System can adapt its established components for different brands and touchpoints.
Each theme is a translation of a brand's corporate design into the Solid DS – not a 1:1 reproduction, but the closest possible approximation within a consistent, scalable system. Certain design decisions are defined system-wide to ensure quality, consistency, and accessibility across all brands.
Unlike a Whitelabel System – where each brand builds and maintains its own library from a blank foundation – the Solid DS retains a shared, opinionated base and adapts only the visual layer.

Regardless of the theme – the proven Solid DS quality remains intact. Structure, logic, accessibility, and maintainability are secured across the entire system. The theme determines how components look. The Solid DS determines how well they work.

Recommendation for Placement: https://solid-design-system.fe.union-investment.de/docs/?path=/docs/packages-components-installation--docs#theme&globals=theme:UI+Light