Update navigation-structure.md

Reinstated the collapsible TOC at the top, to support the reference to it right at the end of the file. (The `TOC` feature can only be used once per page, so this is the only way of illustrating the rendering of the collapsible TOC in the docs.)

docs/navigation-structure.md

@@ -7,11 +7,14 @@ nav_order: 5
 # Navigation Structure
 {: .no_toc }
 
-## Table of contents
-{: .no_toc .text-delta }
-
+<details open markdown="block">
+  <summary>
+    Table of contents
+  </summary>
+  {: .text-delta }
 1. TOC
 {:toc}
+</details>
 
 ---