fix($core): failed to render <Content /> with dynamic pageKey from current $page.

Author: ulivz

packages/@vuepress/core/lib/client/components/Content.js

@@ -1,4 +1,5 @@
-import { setGlobalInfo } from '@app/util'
+import Vue from 'vue'
+import { setGlobalInfo, getPageAsyncComponent } from '@app/util'
 
 export default {
   props: {
@@ -12,6 +13,14 @@ export default {
     const pageKey = this.pageKey || this.$parent.$page.key
     setGlobalInfo('pageKey', pageKey)
 
+    /**
+     * This is for use cases that render `<Content />`
+     * with dynamic pageKey from current $page.
+     */
+    if (!Vue.component(pageKey)) {
+      Vue.component(pageKey, getPageAsyncComponent(pageKey))
+    }
+
     if (pageKey) {
       return h(pageKey)
     }