[IMP] doc: restore canonical URL

Was added at 958f9106 but lost during design change

closes odoo/odoo#41801

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

doc/_extensions/odoo_ext/layout.html

@@ -26,6 +26,11 @@
   {{ super() }}
 {%- endblock -%}
 
+{%- block linktags -%}
+    <link rel="canonical" href="{{ canonical }}" />
+  {{ super() }}
+{%- endblock -%}
+
 {%- block sidebar1 -%}{%- endblock -%}
 {%- block sidebar2 -%}{%- endblock -%}
 {%- block relbar1 -%}{%- endblock -%}