move visibility docs and add .visible mention

docs/_data/nav.yml

@@ -59,7 +59,6 @@
     - title: Display
     - title: Flexbox
     - title: Image replacement
-    - title: Invisible content
     - title: Position
     - title: Responsive helpers
     - title: Screenreaders
@@ -67,6 +66,7 @@
     - title: Spacing
     - title: Typography
     - title: Vertical align
+    - title: Visibility
 
 - title: Extend
   pages: