This commit switches Hugo to a webhook-based building process, with
support for Github, Gitlab, and Gitea hooks (including local versions of
Gitea) initially. In addition, Hugo-based sites are now intended to be
served under a single volume, with ingress configuration pointing to
sub-paths into the volume.
Documentation for webhook setup and NGINX proxy configuration is still
underway, and will be filled in later.
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).
