Add MANIFEST files from 10.3-RELEASE.

The MANIFEST files were obtained by
(a) verifying the signature on the 10.3-RELEASE announcement against
the GPG key in the FreeBSD documentation repository;
(b) downloading all of the bootonly.iso.xz ISOs;
(c) verifying those files against the hashes listed in the signed release
announcement; and
(d) extracting the MANIFEST files.