Fix justified nav styles and update nav docs for fill/justify

- Justified and fill nav variants need .nav-item classes on the anchors in addition to .nav-link
- Fix broken styles for the .nav-justified class which wasn't actually doing what we needed it to

scss/_nav.scss

@@ -99,7 +99,8 @@
 
 .nav-justified {
   .nav-item {
-    flex: 1 1 100%;
+    flex-grow: 1;
+    flex-basis: 0;
     text-align: center;
   }
 }