docs/devel: add some notes on the binfmt-image-debian targets

We document some of this on the wiki but lets move it into our
official developer notes documentation.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-3-alex.bennee@linaro.org>