Add an inline table of contents for small screens

This ToC is only shown when the screen is too small for the sticky sidebar ToC.

We use a `<!-- toc -->` comment and the `replace` function to allow inserting the ToC at well defined points of the page. We don't use the shortcode feature of Zola because of https://github.com/getzola/zola/issues/584.

blog/templates/second-edition/page.html

@@ -34,7 +34,7 @@
             </li>{% endfor %}
         </ol>
     </aside>
-    {{ page.content | safe }}
+    {{ page.content | safe | replace(from="<!-- toc -->", to=macros::toc(page=page))}}
 
     <div class="post-footer-support">
         <h2>Support Me</h2>