Update spacer utilities (#22123)

* Drop -x and -y as they're all the same
- Also move -width to elsewhere in the vars because it makes no sense by spacers.
- Update values of -x and -y across main Sass and docs Sass.
* Update docs to reflect changes; link to spacing utils from options page

scss/mixins/_nav-divider.scss

@@ -4,7 +4,7 @@
 
 @mixin nav-divider($color: #e5e5e5) {
   height: 1px;
-  margin: ($spacer-y / 2) 0;
+  margin: ($spacer / 2) 0;
   overflow: hidden;
   background-color: $color;
 }