NetBSD CI is disabled because it is not yet supported in
zig-bootstrap. Once NetBSD has proper zig-bootstrap support, it can be
re-enabled.
Windows is not solved here yet; will be pushing a separate commit for
that.
* remove unused download page html. It's now handled in the
www.ziglang.org website repo.
* add netbsd to the downloads index.json file that we send to
the www.ziglang.org website repo.
* shallow clone the website repo to avoid downloading old copies of
data.js unnecessarily.
