- site.yaml keys site_url and site_label, validated: http(s) URL with a host
or a path starting with a single /; a label needs a URL
- docs:build and docs:serve flags --site-url and --site-label override them
the way --base-url overrides base_url
- every page header, the 404 page included, links back with the explicit
label, else the URL host, else Home; unset output is unchanged
- docs/console/utilities.md documents the keys and flags