fix: add trailing slash /docs/ to vercel.json redirects

vercel.json

@@ -14,6 +14,10 @@
         {
             "source": "/docs",
             "destination": "/docs/items/nadpis/introduction"
+        },
+        {
+            "source": "/docs/",
+            "destination": "/docs/items/nadpis/introduction"
         }
     ]
 }