Document engine and game internals

[Sheph]

Right now there's almost no documentation on how engine works, need to describe:

• Object and component model (i.e. Scene, SceneObject, Component classes, etc.)
• Level, object and scene instances loading and processing
• Graphics, Lighting, Animation, Particle Effects and Audio, at least short summary
• Configuration file processing, user data
• Input Manager, Controller Context, etc.
• Scripting (there're some docs that are generated by ldoc on game build, but these are somewhat outdated, need to update)
• Something else...