[RFC] Rewrite of DRS and SLP documentation files.

I spent some time rewriting the drs and slp format files.
DRS file is basically done, i think. But the SLP file needs some work - I'm not convinced by the SDL Command section, it seems overly verbose and repeats information

All comments appreciated
https://github.com/Tapsa/genieutils/blob/master/src/resource/DrsFile.cpp#L115
https://github.com/Tapsa/genieutils/blob/master/src/resource/DrsFile.cpp#L88
https://github.com/Tapsa/genieutils/blob/master/include/genie/Types.h#L25