[IMP] doc: define a layout for 'category index' pages

These pages are not directly accessible through other doc’s pages but
there are still indexed by google.

eg.https://www.odoo.com/documentation/9.0/tutorials.html

_extensions/odoo/static/style.less

@@ -742,6 +742,15 @@ article.doc-body {
       .lead;
     }
   }
+
+  &.index-category {
+    min-height: 300px;
+    min-height: 30vh;
+
+    li.toctree-l1 {
+      padding: 5px 0;
+    }
+  }
 }
 
 .content-switcher {