Add a docsref shortcode.

This allows for the build to fail if a reference isn't found.

site/content/docs/4.3/examples/navbar-fixed/index.html

@@ -33,6 +33,6 @@ extra_css:
   <div class="jumbotron">
     <h1>Navbar example</h1>
     <p class="lead">This example is a quick exercise to illustrate how fixed to top navbar works. As you scroll, it will remain fixed to the top of your browser’s viewport.</p>
-    <a class="btn btn-lg btn-primary" href="/docs/{{< param docs_version >}}/components/navbar/" role="button">View navbar docs &raquo;</a>
+    <a class="btn btn-lg btn-primary" href="{{< docsref "/components/navbar" >}}" role="button">View navbar docs &raquo;</a>
   </div>
 </main>