Organize the sections of content within Admin > Appearance

What does this MR do and why?

Within !137942 (merged) we figured out that the settings are not organized properly and some are missing guideline texts. So let's improve that together by e.g., using the good old ABC.

Related issues and MRs:

• Organize the sections of content within Admin >... (#433527 - closed)
• Follow-up from "Add option to add custom profil... (#215650)
• Add option to add custom profile image guidelines (!29894 - merged)
• Add member guideline to admin appearance settings (!137942 - merged)

🛠 with ❤ at Siemens

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