Add migration doc link (#1324)

Author: Celia18305

docs/en-us/toc.yml

@@ -294,3 +294,9 @@
     href: /docs/en-us/exchange/transaction.md
   - name: Distributing GAS to users
     href: /docs/en-us/exchange/gas.md 
+- name: Cross-chain Migration Guide
+  items:
+  - name: Migration Development
+    href: /tutorials/en-us/migrationB.md
+  - name: Migration Guide on Migration Page
+    href: /tutorials/en-us/migration-guide.md

docs/zh-cn/toc.yml

@@ -294,4 +294,10 @@
   - name: 处理资产交易
     href: /docs/zh-cn/exchange/transaction.md
   - name: 给用户分发 GAS
-    href: /docs/zh-cn/exchange/gas.md 
+    href: /docs/zh-cn/exchange/gas.md 
+- name: 跨链迁移指南
+  items:
+  - name: 迁移开发
+    href: /tutorials/zh-cn/migrationB.md
+  - name: 迁移指南
+    href: /tutorials/zh-cn/migration-guide.md