Demonstrate <details> and <summary> tags in doc/user/markdown.md

doc/user/markdown.md

@@ -596,6 +596,30 @@ See the documentation for HTML::Pipeline's [SanitizationFilter](http://www.rubyd
   <dd>Does *not* work **very** well. Use HTML <em>tags</em>.</dd>
 </dl>
 
+#### Details and Summary
+
+Content can be collapsed using HTML's [`<details>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details) and [`<summary>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/summary) tags. This is especially useful for collapsing long logs so they take up less screen space.
+
+<p>
+<details>
+<summary>Click me to collapse/fold.</summary>
+These details will remain hidden until expanded.
+
+<pre><code>PASTE LOGS HERE</code></pre>
+</details>
+</p>
+
+**Note:** Unfortunately Markdown is not supported inside these tags, as described by the [markdown specification](https://daringfireball.net/projects/markdown/syntax#html). You can work around this by using HTML, for example you can use `<pre><code>` tags instead of [code fences](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/markdown.md#code-and-syntax-highlighting).
+
+```html
+<details>
+<summary>Click me to collapse/fold.</summary>
+These details will remain hidden until expanded.
+
+<pre><code>PASTE LOGS HERE</code></pre>
+</details>
+```
+
 ### Horizontal Rule
 
 ```