Update documentation tabs

[alizedebray]

The new documentation tabs section the pages and make the guidelines somewhat difficult to read:
https://design-system.post.ch/?path=/docs/cfa21cbc-5f8d-4a42-8165-7f6ed20bf133--docs

Check if we can update the styles to improve this.

Tasks

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1. Remove lines to the edge of the page
2. Remove dotted lines right and bottom