Refactor site structure with Sidebar Topics Dropdown Plugin

[trueberryless]

This PR refactores all pages in order to use the Starlight Sidebar Topic Dropdown Plugin.

Additionally there is a page decision change: All group links in the sidebar get a new index.mdx page which is not visible in the sidebar but gives a general overview (further discussion and investigation needed whether to show these pages or not).

• Create index.mdx pages for general overview (these files are not shown in sidebar). -> create dynamically or append content if index.mdx exists
• Copy and paste all contents into separate files for more granular structure (less huge pages, more small ones).
• Create sidebar configuration within plugin config.
• Decide whether to show the index.mdx pages as first children in group (Overview routes). -> don't show, automatically generated

I have decided to add the automatically generated index.mdx pages later.

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Latest commit: 5e43407

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