Add memory map reference for each system

[rzumer]

Similar to the Game Identification page, it would be useful to document how memory is mapped for each RA-supported system, especially for implementing support in future clients (libretro cores, etc.).

I only know about these:
GB/C: http://gameboy.mongenel.com/dmg/asmmemmap.html (according to @Jamiras) PC88: http://www.maroon.dti.ne.jp/youkan/pc88/memmap.html -> RA maps main RAM (left column, inc. text window) immediately followed by TVRAM (text VRAM, AKA high-speed RAM). ROM, GVRAM, and extension RAM are not mapped.