[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
I was under the impression that this did not vary, even for AoE and Star Wars: Galactic Battlegrounds (except for year). Certainly, all DRS tools work with AoE and SWGB drs files