feat: improve page component (#5013)

* feat: `page` component support fixed header

* docs: `page`  component documentation

* docs: Improve `props` types of `page`

* docs: improve `fixedHeader` description of `page`

* fix: `page` header border color with fixedHeader

* feat: add `headerClass` for `Page`

playground/src/views/examples/modal/index.vue

@@ -77,6 +77,7 @@ function openFormModal() {
 <template>
   <Page
     description="弹窗组件常用于在不离开当前页面的情况下，显示额外的信息、表单或操作提示，更多api请查看组件文档。"
+    fixed-header
     title="弹窗组件示例"
   >
     <template #extra>