Update docs for new XXL grid tier

- Updates mentions of number and exact tiers
- Updates grid example to include xxl container
- Adds some scss-docs references
- Cleans up other grid mentions and docs
- Updates navbar example to include an expand at XXL variant

site/assets/scss/docs.scss

@@ -37,6 +37,7 @@
 @import "content";
 @import "skippy";
 @import "sidebar";
+@import "layout";
 @import "toc";
 @import "footer";
 @import "component-examples";