Enhance documentation on screen modes and memory layout

[indigodarkwolf]

Had a conversation with a user on Discord after they complained about the difficulty in finding documentation regarding the proper way to VPOKE memory for different SCREEN modes. The heart of their problem seemed to be understanding that the tilemaps from SCREEN all start at $1B000, followed by understanding the VERA memory layout for those tilemaps.

[stople]

Could it be an idea to also add a link to the memory map, instead of only mentioning their start addresses without any context? https://github.com/X16Community/x16-docs/blob/master/X16%20Reference%20-%2009%20-%20VERA%20Programmer's%20Reference.md#vram-address-space-layout

[indigodarkwolf]

How does that look?

[stople]

Looks good. One thing that may be still missing though, is which layer is used for bitmap and charset by default. E.g layer 0, rendered first, with 320x240 bitmap, and layer 1 with a 80x60 (128x64) tilemap for characters, rendered after and on top of bitmap, below sprites. (This is probably not accurate, please replace with the correct explanation). This could e.g be added to the vram memory map, and/or to the SCREEN command documentation. Sorry for feature-creeping your documentation effort though, I guess this can also be added later if relevant.