Make scrollable area keyboard focusable

Author: colinbm

lib/source/layouts/core.erb

@@ -53,7 +53,7 @@
           </div>
         <% end %>
 
-        <div class="app-pane__content toc-open-disabled">
+        <div class="app-pane__content toc-open-disabled" tabindex="0">
           <main id="content" class="technical-documentation" data-module="anchored-headings">
             <%= yield %>
             <%= partial "layouts/page_review" %>