For Windows documentation, "folder" is the correct term. For generic
documentation, probably either would do but I suspect people are
slightly more likely to be familiar with "folder".
Also, moved the "you should install the GBA BIOS" advice from the
install pages (where I doubt anybody would see it) to the "importing and
playing" guide, which I figure people are more likely to stumble across.
The actually instructions are still in `install/general.md` for lack of
anywhere more appropriate.
The existing documentation was getting *way* too long to be a single
document.
I've just bulk moved the existing content into the new structure, but it
definitely needs another pass to make the prose match, fix hyperlinks,
etc. etc.
Tim Allen