Write a "porting SDL to new platforms" page

[icculus]

I wrote this for SDL2 at some point: https://discourse.libsdl.org/t/port-sdl-2-0-to-bios/25453/2

That could probably be cleaned up and put at https://wiki.libsdl.org/SDL2/PortingToNewPlatforms, but whether we do that or not, someone should update that for SDL3 and put that on the wiki.

[icculus]

(This actually might be better in docs/README-porting.md or something, which will bridge to the wiki anyhow.)