Rewrite grid docs to account for new grid tier

- Breaks grid content across multiple pages
- Updates mentions of grid tiers from five to six
- Rewrites how it works sections to reference new options and tiers

site/data/sidebar.yml

@@ -14,9 +14,14 @@
 
 - title: Layout
   pages:
-    - title: Overview
+    - title: Breakpoints
+    - title: Containers
     - title: Grid
-    - title: Utilities for layout
+    # - title: Rows
+    - title: Columns
+    - title: Gutters
+    - title: Utilities
+    - title: Z-index
 
 - title: Content
   pages: