feat: add welcome v2 docs (#860)

Author: finalchild

src/routes/(root)/opi/ko/_nav.yaml

@@ -24,6 +24,7 @@
         - /ko/integration/pg/v2/inicis-jp
         - /ko/integration/pg/v2/ksnet
         - /ko/integration/pg/v2/kcp-v2
+        - /ko/integration/pg/v2/welcome
         - /ko/integration/pg/v2/kakaopay
         - /ko/integration/pg/v2/naverpay
         - /ko/integration/pg/v2/tosspay-v2