document "hideTitle" prop (#4826)

The Banner "overview" docs mention the ability to hide the title visibly (see https://primer.style/components/banner#title), but how to actually implement this in the React component isn't documented. This change adds the correct prop, `hideTitle`, to the documented props for Banner.

packages/react/src/Banner/Banner.docs.json

@@ -16,6 +16,11 @@
       "type": "React.ReactNode",
       "description": "Provide an optional description for the Banner. This should provide supplemental information about the Banner"
     },
+    {
+      "name": "hideTitle",
+       "type": "boolean",
+      "description": "Whether to hide the title visually."
+    },
     {
       "name": "icon",
       "type": "React.ReactNode",