UX: Display tag's description as title in navigation menu (#22710)

Why this change?

We're already displaying a category's description as the title attribute
on the category section link. We should do the same for tags as well.

spec/requests/api/schemas/json/site_response.json

@@ -800,6 +800,9 @@
     },
     "denied_emojis" : {
       "type": "array"
+    },
+    "navigation_menu_site_top_tags": {
+      "type": "array"
     }
   },
   "required": [