Make imports/exports not be a dropdown in sidebar (#19907)

* Don't make a section be a dropdown if it has no child links

* - Cleaned up code

- Updated tests

* Document what the 'if' is checking

nimdoc/testproject/expected/testproject.html

@@ -51,12 +51,7 @@
     </div>
     <ul class="simple simple-toc" id="toc-list">
   <li>
-  <details open>
-    <summary><a class="reference reference-toplevel" href="#6" id="56">Imports</a></summary>
-    <ul class="simple simple-toc-section">
-      
-    </ul>
-  </details>
+  <a class="reference reference-toplevel" href="#6" id="56">Imports</a>
 </li>
 <li>
   <details open>