Hey Radik -
No. The XML schema of workbooks and data sources are not documented - "hacking" these are actually not supported, although people do so.
Also, if you edit your workbooks (via text editor, Java, Python, etc.) and break things then my understanding is that Tableau's policy is that you won't get support.
I'm not sure how to address this.
On the one hand it would be helpful if the structure and semantics for Tableau's various files were documented; on the other it probably wouldn't help all that much since one would have to spend a lot of time, energy, and effort into understanding the documentation and relating it to one's actual files.
And on the other, other hand, there are many, many other things that Tableau should document before the physical layout of its files. For example, there's no official documentation on the mechanisms by which Tableau accesses and processes data in it various scenarios, so much of what we know about how Tableau works and consequently how to use it deliberately comes from the work Jonathan and others have put into ferreting out Tableau's secrets and communicating them to the community.
So... the larger point is that it would be very, very helpful for Tableau to provide real, useful documentation about its models and operations, across the board, making it easier to use effectively.