Add 'child_nav_order' front matter to be able to sort navigation pages in reverse (#726)

Set `child_nav_order` to `desc` to reverse the sort order for a child section.

Co-authored-by: Matt Wang <matt@matthewwang.me>

docs/navigation-structure.md

@@ -145,6 +145,21 @@ nav_order: 2
 
 The Buttons page appears as a child of UI Components and appears second in the UI Components section.
 
+### Ordering child pages
+
+You can optionally add the following to the YAML front matter to change the default sort order of child pages from ascending to descending order:
+
+- `child_nav_order: desc`
+
+#### Example
+{: .no_toc }
+```yaml
+---
+layout: default
+title: Descending Child Pages
+child_nav_order: desc
+---
+
 ### Auto-generating Table of Contents
 
 By default, all pages with children will automatically append a Table of Contents which lists the child pages after the parent page's content. To disable this auto Table of Contents, set `has_toc: false` in the parent page's YAML front matter.