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/content/docs/4.3/layout/containers.md

@@ -153,15 +153,7 @@ Use `.container-fluid` for a full width container, spanning the entire width of
 
 As shown above, Bootstrap generates a series of predefined container classes to help you build the layouts you desire. You may customize these predefined container classes by modifying the Sass map (found in `_variables.scss`) that powers them:
 
-{{< highlight scss >}}
-$container-max-widths: (
-  sm: 540px,
-  md: 720px,
-  lg: 960px,
-  xl: 1140px,
-  xxl: 1320px
-) !default;
-{{< /highlight >}}
+{{< scss-docs name="container-max-widths" file="scss/_variables.scss" >}}
 
 In addition to customizing the Sass, you can also create your own containers with our Sass mixin.