Feature/update theming page

[cbraehmig]

This pull request updates the theming documentation to clarify concepts, improve accuracy, and simplify explanations. The main changes include refining the definition of a theme, updating terminology for consistency, and revising the state diagram for better readability.

Documentation improvements:

• Updated the introductory explanation of what a theme is, focusing on its role as an implementation of a central design template in KoliBri.
• Clarified that CSS is decoupled from components (using the correct German term "Komponenten") and not tied to the English "Components".
• Simplified the explanation of component theming, removing references to "Mandant" and focusing on theme-level customization.

State diagram and process improvements:

• Revised the Mermaid state diagram to use clearer labels, improved formatting, and more accurate process steps (e.g., replacing "Designer" with "Theme", cleaning up notes, and adjusting transitions).
• Updated the description of the design-to-theme workflow to remove references to the KoliBri Designer and clarify how CSS is applied to base components.

[github-actions]

🚀 Preview deployed to GitHub Pages: https://public-ui.github.io/public-ui.github.io/pr-797/9fe8614/
Commit: 9fe8614

[github-actions]

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