feat: add canonical URL meta tag to improve SEO (#556)

Prevents Google from choosing /en/latest/ as canonical URL,
helping to resolve duplicate content issues across versions.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: thephez

_templates/layout.html

@@ -1,5 +1,9 @@
 {% extends "!layout.html" %}
 {% block extrahead %}
+    {# Canonical URL: stops Google choosing /en/latest/ as canonical #}
+    {% if canonical_url %}
+    <link rel="canonical" href="{{ canonical_url }}" />
+    {% endif %}
     <link href="{{ pathto("_static/style.css", True) }}" rel="stylesheet" type="text/css">
     <meta name="google-site-verification" content="JVzTMPjsCY-U-SaJERGf-eviRxXUPfFkPFdiF_MINCU" />
     <meta name="author" content="Dash Core Group, Inc." />