Document VO application via Plony... and some structure simplification / deduplication

• Primary goal: Changing how-tos towards VO application via plony, as decided in AAI meeting 2025-04-16.
• This triggered several follow-ups to unify the intro pages and some of the how-tos, as otherwise the plony description would interfere with other legacy parts
  • related: !654 (comment 6827929)

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• Edit here
• Review here | Old version