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/load_test.go

@@ -27,11 +27,20 @@ func TestParseSite(t *testing.T) {
 		{"reserved index", "title: t\ndescription: d\nsections:\n  - name: index\n    title: I\n", `section name "index" is reserved`},
 		{"bad slug", "title: t\ndescription: d\nsections:\n  - name: Setup\n    title: S\n", `section name "Setup" must be lowercase`},
 		{"duplicate", "title: t\ndescription: d\nsections:\n  - name: a\n    title: A\n  - name: a\n    title: B\n", `section "a" is listed twice`},
+		{"site_url javascript", valid + "site_url: javascript:alert(1)\n", "site_url must be an http(s) URL with a host or a path starting with a single /"},
+		{"site_url protocol-relative", valid + "site_url: //acme.example\n", "site_url must be an http(s) URL with a host or a path starting with a single /"},
+		{"site_label without site_url", valid + "site_label: Acme\n", "site_label needs site_url"},
 	} {
 		if _, err := ParseSite([]byte(tc.raw)); err == nil || !strings.Contains(err.Error(), tc.want) {
 			t.Errorf("%s: err = %v, want %q", tc.name, err, tc.want)
 		}
 	}
+	for _, u := range []string{"https://acme.example/", "/"} {
+		got, err := ParseSite([]byte(valid + "site_url: " + u + "\n"))
+		if err != nil || got.SiteURL != u {
+			t.Errorf("site_url %q: ParseSite = %+v, %v", u, got, err)
+		}
+	}
 	if strings.Contains(func() string { _, err := ParseSite([]byte(valid + "colour: red\n")); return err.Error() }(), "\n") {
 		t.Error("a decode error must be reported on one line")
 	}