The file was originally named `higan-config.md` because the dialog box
it described was called "higan Configuration", which was because the
menu-item to open it was in a menu called "Settings" and it would have
been weird to have a "Settings" item in a "Settings" menu.
Now there are individual menu-items for each tab in the Settings dialog,
none of which are named Settings, so the dialog has been renamed back to
"Settings", and the filename should match.
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.