move flexbox to getting started

docs/_data/nav.yml

@@ -10,6 +10,7 @@
     - title: JavaScript
     - title: Compiling
     - title: Customize
+    - title: Flexbox
     - title: Support
     # - title: Third party support
     - title: Accessibility
@@ -40,7 +41,6 @@
         - title: Viewports
         - title: Print
         - title: Test cases
-    - title: Flexbox
 
 - title: Components
   pages: