In addition to changes made when Git repositories update, Hugo is able
to watch for timed changes to content (say, for content that has a
publication date in the future) and build these automatically.
This commit switches from a plain `hugo build` to a `hugo watch` process
to facilitate these use-cases.
By default, a `hugo-build-local@.service` systemd service is included,
for building Hugo-based sites as stored in a local Git repository (as
served by the `git` service), and into a volume (which is intended to be
served by the `nginx-serve-volume` service).
