docs: added a redirect from the old billing docs page to the new knowledge base section (#10817)

# Which Problems Are Solved

Added a redirect from the old billing docs page to the new knowledge
base section.

docs/vercel.json

@@ -218,6 +218,11 @@
       "source": "/docs/guides/integrate/login/login-users#password-reset",
       "destination": "/docs/guides/integrate/login/hosted-login#password-reset",
       "permanent": true
+    },
+    {
+      "source": "/docs/legal/service-description/billing",
+      "destination": "https://help.zitadel.com/pricing-and-billing-of-zitadel-services",
+      "permanent": true
     }
   ]
 }