feat(docsite): optional site_url and site_label link back to the main site

- 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

internal/docsite/docsite.go

@@ -34,6 +34,10 @@ type Options struct {
 	Out string
 	// BaseURL overrides site.yaml base_url when non-empty.
 	BaseURL string
+	// SiteURL overrides site.yaml site_url when non-empty.
+	SiteURL string
+	// SiteLabel overrides site.yaml site_label when non-empty.
+	SiteLabel string
 	// Commands is the set of summer and application command names pages
 	// may show. Check and Build report a problem when it is nil.
 	Commands *Commands