German online help for 8.6.0

- OXUD-1112 Update the slightly outdated topic 'Daten verwalten mit Ordnern'
- OXUD-1113 Update: Button for open/close all mails of a conversation has been removed in UI 8
- OXUD-1114 Update and fix the outdated topic 'Daten teilen mit Hilfe von Freigaben'
- OXUD-1117 Improve the description of the mail folder view
- OXUD-1118 Add a new topic describing the Jitsi integration
- OXUD-1122 Fix the instruction of searching for tasks
- OXUD-1123 Update the icon for 'All applications' and the icons of the App Launcher