Harmonize content related to ios-app

[mmattel]

References: owncloud/ios-app#1094 ([docs/folder-structure] Changed Docs Structure / MDM Fixes)

Als see comment: owncloud/ios-app#1094 (comment)

Filename casing of doc/CONFIGURATION.md should be equal to the referenced file in the ios-app repo and the link in doc/CONFIGURATION.md:18 should be corrected to the valid address used in the referenced PR from above.

@hosy fyi