Restore grid offset classes

Fixes #23360 by restoring just the offset class generation to our grid
framework mixins. Also restores the `make-col-offset` mixin.

Docs have been restored to illustrate this behavior and merged with the
newer margin utilities examples.

scss/mixins/_grid.scss

@@ -45,3 +45,7 @@
   // do not appear to require this.
   max-width: percentage($size / $columns);
 }
+
+@mixin make-col-offset($size, $columns: $grid-columns) {
+  margin-left: percentage($size / $columns);
+}