G-API: Doxygen class reference
* G-API Doxygen documentation: covered cv::GComputation
* G-API Doxygen documentation: added sections on compile arguments
* G-API Doxygen documentation: restructuring & more text
* Added new sections (organized API reference into it);
* Documented GCompiled, compile args, backends, etc.
* G-API Doxygen documentation: documented GKernelPackage and added group for meta
