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/_images.scss

@@ -33,7 +33,7 @@
 }
 
 .figure-img {
-  margin-bottom: ($spacer-y / 2);
+  margin-bottom: ($spacer / 2);
   line-height: 1;
 }