Add example collapsible section that is open by default (#34837)

Co-authored-by: Ben Ahmady <32935794+subatoi@users.noreply.github.com>
Co-authored-by: Alex Nguyen <150945400+nguyenalex836@users.noreply.github.com>

content/get-started/writing-on-github/working-with-advanced-formatting/organizing-information-with-collapsed-sections.md

@@ -43,6 +43,11 @@ After a reader clicks {% octicon "triangle-right" aria-label="The right triangle
 
 ![Screenshot of the Markdown above on this page as rendered on {% data variables.product.prodname_dotcom %}, indicating that a collapsed section can contain headers, sentences of text, images, and code blocks.](/assets/images/help/writing/open-collapsed-section.png)
 
+Optionally, to make the section display as open by default, add the `open` attribute to the `<details>` tag:
+
+```html
+<details open>
+
 ## Further reading
 
 * [{% data variables.product.prodname_dotcom %} Flavored Markdown Spec](https://github.github.com/gfm/)