Improve aria-labels for example icon buttons and add guidance (#4191)

* Improve aria-labels for example icon buttons and add guidance

* test(vrt): update snapshots

---------

Co-authored-by: joshblack <[email protected]>

Author: owenniblock

.playwright/snapshots/components/IconButton.test.ts-snapshots/IconButton-Playground-dark-colorblind-linux.png

@@ -35,6 +35,12 @@
       "defaultValue": "",
       "description": "provide an octicon. It will be placed in the center of the button"
     },
+    {
+      "name": "aria-label",
+      "type": "string",
+      "defaultValue": "",
+      "description": "Use an aria label to describe the functionality of the button. Please refer to [our guidance on alt text](https://primer.style/guides/accessibility/alternative-text-for-images) for tips on writing good alternative text."
+    },
     {
       "name": "sx",
       "type": "SystemStyleObject"