docs(upgrade-guides): Add the v8 upgrade guide to versioned docs (#12086)

Author: Tobbe

docs/versioned_docs/version-8.0/upgrade-guides/v8.md

@@ -323,6 +323,16 @@
           "dirName": "how-to"
         }
       ]
+    },
+    {
+      "type": "category",
+      "label": "Upgrade Guide",
+      "items": [
+        {
+          "type": "autogenerated",
+          "dirName": "upgrade-guides"
+        }
+      ]
     }
   ]
 }