services: guix-publish: Add zstd compression by default.

That way, the default config with (advertises? #t) and without a cache
will offer zstd-compressed substitutes, which should lead to much higher
throughput than gzip.

* gnu/services/base.scm (default-compression): When
'guix-publish-configuration-cache' returns true, use higher level
compression ratios.  Add "zstd".
* doc/guix.texi (Base Services): Mention zstd.