Allow overriding toc levels at the guide level

For reference guides, having a deeper TOC might make sense, typically
for the Qute reference guide.

Author: gsmet

_layouts/guides.html

@@ -59,7 +59,7 @@ <h1 class="text-caps">{{page.title}} {{page.docversion}}</h1>
       {{ content }}
     </div>
     <div class="grid__item width-4-12 width-12-12-m tocwrapper">
-      <div class="hide-mobile toc">{{ page.document | tocify_asciidoc: 2 }}</div>
+      <div class="hide-mobile toc">{{ page.document | tocify_asciidoc }}</div>
     </div>
   </div>
   {% if relations and relations[guide_url] -%}