docs(de): translation batch

[domsteindl]

Added german translation for the discord server verification, the vtc discord role (verified) and the livery guidelines
For a few pieces of text i wasn't sure if i should translate them (e. g. error messages like "You must own a verified VTC") so i left them in english. If needed just feel free to tell me and i'll make the changes accordingly

• The frontmatter is preserved and translated
• All headings are translated
• All body text is translated
• Links point to the correct locale
• Code blocks and URLs are unchanged
• Sidebar labels are updated in astro.config.mjs
• The page renders correctly in the dev server
• No broken formatting or Markdown errors
• Terminology is consistent throughout

[olususus]

Thank you for contributing!