Fix linkified breadcrumb items in readthedocs theme

A doc can never have a `link` attribute so this branch never kicked in.

Actually even now, this branch will still not kick in, only mkdocs-section-index makes this state possible.

mkdocs/themes/readthedocs/breadcrumbs.html

@@ -3,8 +3,8 @@
     <li><a href="{{ nav.homepage.url|url }}" class="icon icon-home" aria-label="{% trans %}Docs{% endtrans %}"></a></li>
     {%- if page %}
       {%- for doc in page.ancestors[::-1] %}
-        {%- if doc.link %}
-          <li class="breadcrumb-item"><a href="{{ doc.link|e }}">{{ doc.title }}</a></li>
+        {%- if doc.url %}
+          <li class="breadcrumb-item"><a href="{{ doc.url|url }}">{{ doc.title }}</a></li>
         {%- else %}
           <li class="breadcrumb-item">{{ doc.title }}</li>
         {%- endif %}