How can I read iTunes backup data? Is there any official documentation on how I can read out a non-encrypted iTunes backup of an iOS device?

I researched but unfortunately I didn't find an answer.
No - Apple does not document the storage format for iTunes backup of iOS data.

It's been reverse engineered by several companies, though. You can search for iTunes backup extractor and all manor of forensic software suites that companies and governments buy to extract data during investigations.