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/static/css/main.css

@@ -172,6 +172,10 @@ aside#all-posts-link {
 }
 
 @media (min-width: 80rem) {
+  #toc-inline {
+    display: none;
+  }
+
   #toc-aside {
     display: block;
     width: 12rem;