[overview] Split into relevant pieces

[dbosk]

Make overview a true overview of trusted computing. The parts that should be moved:

• [x] DRM should be made a separate learning session (#7).
• [ ] Secure Protocols should be made it's own module. [Is there such a part here?]
• [x] Information Hiding belongs to a steganography module, or together with DRM (#6, #5).