implemented the hackmd for not found page

resource/locales/en_US/translation.json

@@ -439,6 +439,7 @@
   "hackmd": {
     "hack_md": "HackMD",
     "not_set_up": "HackMD is not set up.",
+    "used_for_not_found": "Can not use HackMD to a page that does not exist.",
     "start_to_edit": "Start to edit with HackMD",
     "clone_page_content": "Click to clone page content and start to edit.",
     "unsaved_draft": "HackMD has unsaved draft.",
@@ -453,7 +454,8 @@
     "not_initialized": "HackmdEditor component has not initialized",
     "someone_editing": "Someone editing this page on HackMD",
     "this_page_has_draft": "This page has a draft on HackMD",
-    "need_to_associate_with_growi_to_use_hackmd_refer_to_this": "To use HackMD for simultaneous multi-person editing, need to associate HackMD with GROWI.Please refer to <a href='https://docs.growi.org/en/admin-guide/admin-cookbook/integrate-with-hackmd.html'>here</a>."
+    "need_to_associate_with_growi_to_use_hackmd_refer_to_this": "To use HackMD for simultaneous multi-person editing, need to associate HackMD with GROWI.Please refer to <a href='https://docs.growi.org/en/admin-guide/admin-cookbook/integrate-with-hackmd.html'>here</a>.",
+    "need_to_make_page": "To use HackMD, please make a new page."
   },
   "slack_notification": {
     "popover_title": "Slack Notification",

resource/locales/ja_JP/translation.json

@@ -441,6 +441,7 @@
   "hackmd":{
     "hack_md": "HackMD",
     "not_set_up": "HackMD はセットアップされていません",
+    "used_for_not_found": "存在しないページでHackMDを利用することはできません",
     "start_to_edit": "HackMD を開始する",
     "clone_page_content": "ページを複製して編集を開始します",
     "unsaved_draft": "HackMD のドラフトが保存されていません",
@@ -455,7 +456,8 @@
     "not_initialized": "HackMD コンポーネントは初期化されていません",
     "someone_editing": "このページは、HackMD で編集されています。",
     "this_page_has_draft": "このページは、HackMD のドラフトがあります。",
-    "need_to_associate_with_growi_to_use_hackmd_refer_to_this": "HackMD を利用して同時多人数編集を行うには、HackMD と GROWI を連携する必要があります。<a href='https://docs.growi.org/ja/admin-guide/admin-cookbook/integrate-with-hackmd.html'>こちら</a>を参照してください。"
+    "need_to_associate_with_growi_to_use_hackmd_refer_to_this": "HackMD を利用して同時多人数編集を行うには、HackMD と GROWI を連携する必要があります。<a href='https://docs.growi.org/ja/admin-guide/admin-cookbook/integrate-with-hackmd.html'>こちら</a>を参照してください。",
+    "need_to_make_page": "HackMDを利用するためには、新しいページを作成してください。"
   },
   "slack_notification": {
     "popover_title": "Slack 通知",

resource/locales/zh_CN/translation.json

@@ -414,7 +414,8 @@
 	},
 	"hackmd": {
     "hack_md": "HackMD",
-		"not_set_up": "HackMD is not set up.",
+    "not_set_up": "HackMD is not set up.",
+    "used_for_not_found": "Can not use HackMD to a page that does not exist.",
 		"start_to_edit": "Start to edit with HackMD",
 		"clone_page_content": "Click to clone page content and start to edit.",
 		"unsaved_draft": "HackMD has unsaved draft.",
@@ -429,7 +430,8 @@
 		"not_initialized": "HackmdEditor component has not initialized",
 		"someone_editing": "Someone editing this page on HackMD",
     "this_page_has_draft": "This page has a draft on HackMD",
-    "need_to_associate_with_growi_to_use_hackmd_refer_to_this": "若要使用HackMD的多人同时编辑功能，请先关联HackMD和GROWI。详情请参考<a href='https://docs.growi.org/cn/admin-guide/admin-cookbook/integrate-with-hackmd.html'>这里</a>。"
+    "need_to_associate_with_growi_to_use_hackmd_refer_to_this": "若要使用HackMD的多人同时编辑功能，请先关联HackMD和GROWI。详情请参考<a href='https://docs.growi.org/cn/admin-guide/admin-cookbook/integrate-with-hackmd.html'>这里</a>。",
+    "need_to_make_page": "To use HackMD, please make a new page."
   },
   "slack_notification": {
     "popover_title": "Slack Notification",

src/client/js/components/PageEditorByHackmd.jsx

@@ -230,9 +230,9 @@ class PageEditorByHackmd extends React.Component {
     const hackmdUri = this.getHackmdUri();
     const { pageContainer, t } = this.props;
     const {
-      revisionId, revisionIdHackmdSynced, remoteRevisionId,
+      revisionId, revisionIdHackmdSynced, remoteRevisionId, pageId,
     } = pageContainer.state;
-
+    const isPageNotFound = pageId == null;
 
     let content;
 
@@ -248,6 +248,19 @@ class PageEditorByHackmd extends React.Component {
         </div>
       );
     }
+
+    /*
+    * used HackMD from NotFound Page
+    */
+    else if (isPageNotFound) {
+      content = (
+        <div>
+          <p className="text-center hackmd-status-label"><i className="fa fa-file-text"></i> { t('hackmd.used_for_not_found')}</p>
+          {/* eslint-disable-next-line react/no-danger */}
+          <p dangerouslySetInnerHTML={{ __html: t('hackmd.need_to_make_page') }} />
+        </div>
+      );
+    }
     /*
      * Resume to edit or discard changes
      */