code review：补全支付文档

src/views/pay/app/index.vue

 <template>
+  <doc-alert title="支付功能开启" url="https://doc.iocoder.cn/pay/build/" />
   <!-- 搜索 -->
   <ContentWrap>
     <el-form

src/views/pay/demo/index.vue

 <template>
+  <doc-alert title="支付宝支付接入" url="https://doc.iocoder.cn/pay/alipay-pay-demo/" />
+  <doc-alert title="支付宝、微信退款接入" url="https://doc.iocoder.cn/pay/refund-demo/" />
+  <doc-alert title="微信公众号支付接入" url="https://doc.iocoder.cn/pay/wx-pub-pay-demo/" />
+  <doc-alert title="微信小程序支付接入" url="https://doc.iocoder.cn/pay/wx-lite-pay-demo/" />
+
   <!-- 操作工具栏 -->
   <el-row :gutter="10" class="mb8">
     <el-col :span="1.5">

src/views/pay/refund/index.vue

 <template>
+  <doc-alert title="支付宝、微信退款接入" url="https://doc.iocoder.cn/pay/refund-demo/" />
+
   <!-- 搜索工作栏 -->
   <ContentWrap>
     <el-form