The public headers saw lots of cleanups, backporting from SDL3 docs, and
merging with the wiki.
The markdown files in docs/README-*.md were converted to Unix endlines.
- SDL_JoystickGUID -> SDL_GUID (though we retain a type alias)
- Operations for GUID <-> String ops are now in
src/SDL_guid.c and include/SDL_guid.h
- The corresponding Joystick operations delegate to SDL_guid.c
- Added test/testguid.c
