Ship systemd unit files in the Linux static tarballs

[piyush15102003]

Ship the systemd unit files inside the Linux static tarballs

Closes #249.

What

packaging/static/Makefile now copies packaging/systemd/cri-docker.service and
cri-docker.socket into the archive directory before tar runs, for both the static-linux
(amd64) and cross-arm (arm64) targets. The Mac and Windows archives are unchanged since the
units are Linux-only.

The manual install page is updated to say the unit files are in the archive, keeping the
wget from packaging/systemd as the fallback for older releases.

Why

The release .tgz only contained the binary, while the manual-install instructions read as if
the unit files were in it. Anyone installing on an unsupported platform had to go find them in
the source tree separately, which is what #249 reports. The deb/rpm packages already install
these two files; the tarballs were the only distribution that left them out.

How I tested

Ran the real Makefile targets in an Alpine container with make, bash and a stub go on
PATH (so the recipe runs end to end without a 10-minute vendored Go build):

make -C packaging/static static-linux VERSION=0.0.0-test
tar -tzvf packaging/static/build/linux/cri-dockerd-0.0.0-test.amd64.tgz
make -C packaging/static cross-arm VERSION=0.0.0-test
tar -tzf packaging/static/build/arm/cri-dockerd-0.0.0-test.arm64.tgz

Both archives list cri-dockerd/cri-dockerd, cri-dockerd/cri-docker.service and
cri-dockerd/cri-docker.socket; cmp confirms the units are byte-identical to
packaging/systemd/, and they land as 0644 regardless of the checkout's file modes
(install -m 0644, not cp). The Go build step itself is untouched.