templates: page: support external redirects

Some of the docs have moved, so let's send visitors to the new ones (and hide the outdated content).

templates/docs/page.html

@@ -8,22 +8,28 @@
   {# This value is matched by the config.extra.menu.main~url #}
   {% set current_section = page.path | split(pat="/") | slice(end=-2) | join(sep="/") %}
   {{ macros_header::header(current_section=current_section)}}
+  {% if page.extra.external_redirect %}<meta http-equiv="refresh" content="0; URL={{ page.extra.external_redirect }}" />{% endif %}
 {% endblock header %}
 
 {% block content %}
 <div class="wrap container" role="document">
   <div class="content">
     <div class="row flex-xl-nowrap">
 	  {{ macros_sidebar::docs_sidebar(current_section=current_section) }}
-	  {{ macros_toc::docs_toc(page=page) }}
+          {% if not page.extra.external_redirect %}{{ macros_toc::docs_toc(page=page) }}{% endif %}
       <main class="docs-content col-lg-11 col-xl-9">
         <h1>{{ page.title }}</h1>
-        {% if page.extra.lead %}<p class="lead">{{ page.extra.lead | safe }}</p>{% endif %}
-        {{ page.content | safe }}
-        {% if config.extra.edit_page %}
-          {{ macros_edit_page::docs_edit_page(current_path=current_path) }}
+	{% if page.extra.external_redirect %}
+          <p>This documentation has been moved to <a href="{{ page.extra.external_redirect }}">{{ page.extra.external_redirect }}</a>.</p>
+          <p>You should be automatically redirected.</p>
+        {% else %}
+          {% if page.extra.lead %}<p class="lead">{{ page.extra.lead | safe }}</p>{% endif %}
+          {{ page.content | safe }}
+          {% if config.extra.edit_page %}
+            {{ macros_edit_page::docs_edit_page(current_path=current_path) }}
+          {% endif %}
+          {{ macros_navigation::docs_navigation(page=page, current_section=current_section) }}
         {% endif %}
-        {{ macros_navigation::docs_navigation(page=page, current_section=current_section) }}
       </main>
     </div>
   </div>