Update documentation with collapsible TOC

Makes the TOC at the top of the page collapsible.
Adds an explanation of the markup used to produce a collapsible TOC.
Adds a comment about the possibility of using an unordered list for the TOC,
and about using TOC only once per page.

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>
 
 ---
 
@@ -224,4 +227,19 @@ To generate a Table of Contents on your docs pages, you can use the `{:toc}` met
 {:toc}
 ```
 
-This example skips the page name heading (`#`) from the TOC, as well as the heading for the Table of Contents itself (`##`) because it is redundant, followed by the table of contents itself.
+This example skips the page name heading (`#`) from the TOC, as well as the heading for the Table of Contents itself (`##`) because it is redundant, followed by the table of contents itself. To get an unordered list, replace  `1. TOC` above by `- TOC`.
+
+The table of contents can be made collapsible by adding HTML markup, as in the following example. The attribute `open` and the styling with `{: .text-delta }` are optional.
+
+```markdown
+<details open markdown="block">
+  <summary>
+    Table of contents
+  </summary>
+  {: .text-delta }
+1. TOC
+{:toc}
+</details>
+```
+
+The result is shown at [the top of this page](#navigation-structure) (`{:toc}` can be used only once on each page).