Enhance external storage configuration

[AlexAndBear]

WHAT Needs to be Documented?

• /admin_manual/configuration/files/external_storage_configuration_gui should be located at /admin_manual/configuration/files/external_storage/
• Section Setting Up Google Drive and Dropbox Connections and FTP should be removed due duplication
• Image for Enabling External Storage Support should be replaced, this app is not visible in the app overview, instead show the settings thick
• Add an important note for Improve UI success/error while adding storages core#38288

WHY Should This Change Be Made?

consistency and simplicity

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other