[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
The old doc contained a hint about the copyright size being 40 bytes _only_ for the specific case of Age of Empires II. You removed that without any replacement.