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/emit.go

@@ -67,6 +67,11 @@ type pageView struct {
 	Content     template.HTML
 	Assets      string
 	HomeURL     string
+	// SiteURL and SiteLabel are the link back to the main site, empty
+	// when site_url is unset. SiteURL points at another site, so it is
+	// never prefixed with base_url.
+	SiteURL     string
+	SiteLabel   string
 	SearchIndex string
 	LLMS        string
 	LLMSFull    string
@@ -85,6 +90,8 @@ func (s *site) baseView(current *Page) pageView {
 	return pageView{
 		Assets:      s.url("assets"),
 		HomeURL:     s.url("index.html"),
+		SiteURL:     s.siteURL,
+		SiteLabel:   s.siteLabel,
 		SearchIndex: s.url("search-index.json"),
 		LLMS:        s.url("llms.txt"),
 		LLMSFull:    s.url("llms-full.txt"),