Developer documentation should be its own manual. As a start, move all
developer-oriented files to a separate directory.
Also move non-text files to their own directories: docs/config/ for
QEMU -readconfig input, and docs/spin/ for formal models to be used
with the SPIN model checker.
Reviewed-by: Daniel P. Berrange <email@example.com>
Signed-off-by: Paolo Bonzini <firstname.lastname@example.org>