The MANUAL_BUILDDIR directory is automatically created by sphinx-build
for the other targets.  The index.html target does not use sphinx-build
so we must manually create the directory to avoid the following error:

  GEN     docs/built/index.html
  /bin/sh: docs/built/index.html: No such file or directory

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20200120163400.603449-1-stefanha@redhat.com
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>