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/theme/assets/site.css

@@ -280,6 +280,23 @@ a { color: var(--c-primary); }
 
 .wordmark-cms { color: var(--c-accent); }
 
+/* Optional link back to the main site (site_url). */
+.site-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  margin-left: 8px;
+  color: var(--c-side-label);
+  font-size: 14px;
+  line-height: 1.5;
+  text-decoration: none;
+  transition: color var(--ease);
+}
+
+.site-link:hover { color: var(--c-accent); }
+
+.site-link .icon { width: 16px; height: 16px; vertical-align: 0; }
+
 .icon-button,
 .theme-toggle,
 .search-trigger {
@@ -892,6 +909,7 @@ html.js .copy-button:not([hidden]) { display: inline-flex; }
 }
 
 @media (max-width: 639.98px) {
+  .site-link-label { display: none; }
   .pager { grid-template-columns: 1fr; }
   .pager-next { grid-column: 1; }
   .pager-empty { display: none; }