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-framework.scss

@@ -51,6 +51,15 @@
           order: $i;
         }
       }
+
+      // `$columns - 1` because offsetting by the width of an entire row isn't possible
+      @for $i from 0 through ($columns - 1) {
+        @if not ($infix == "" and $i == 0) { // Avoid emitting useless .offset-0
+          .offset#{$infix}-#{$i} {
+            @include make-col-offset($i, $columns)
+          }
+        }
+      }
     }
   }
 }

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);
+}