Docs: Breaks out Utilities into separate section and optimizes its pages (#20678)

docs/utilities/close-icon.md

@@ -0,0 +1,13 @@
+---
+layout: docs
+title: Close icon
+group: utilities
+---
+
+Use a generic close icon for dismissing content like modals and alerts. **Be sure to include text for screen readers**, as we've done with `aria-label`.
+
+{% example html %}
+<button type="button" class="close" aria-label="Close">
+  <span aria-hidden="true">&times;</span>
+</button>
+{% endexample %}