improve the link to the docs for slack integration

apps/app/public/static/locales/en_US/admin.json

@@ -594,7 +594,6 @@
     "slack_integration": "Slack Integration",
     "selecting_bot_types": {
       "slack_bot": "Slack bot",
-      "detailed_explanation": "Detailed explanation",
       "official_bot": "Official bot",
       "custom_bot": "Custom bot",
       "without_proxy": "without proxy",

apps/app/public/static/locales/ja_JP/admin.json

@@ -602,7 +602,6 @@
     "slack_integration": "Slack連携",
     "selecting_bot_types": {
       "slack_bot": "Slack bot",
-      "detailed_explanation": "詳しい説明はこちら",
       "official_bot": "Official bot",
       "custom_bot": "Custom bot",
       "without_proxy": "without proxy",

apps/app/public/static/locales/zh_CN/admin.json

@@ -602,7 +602,6 @@
     "slack_integration": "Slack一体化",
     "selecting_bot_types": {
       "slack_bot": "Slack bot",
-      "detailed_explanation": "详细说明",
       "official_bot": "Official bot",
       "custom_bot": "Custom bot",
       "without_proxy": "without proxy",

apps/app/src/components/Admin/SlackIntegration/SlackIntegration.jsx

@@ -210,9 +210,8 @@ const SlackIntegration = () => {
       <div className="selecting-bot-type mb-5">
         <h2 className="admin-setting-header mb-4">
           {t('admin:slack_integration.selecting_bot_types.slack_bot')}
-          <a className="ml-2 btn-link" href={t('admin:slack_integration.docs_url.slack_integration')} target="_blank" rel="noopener noreferrer">
-            {t('admin:slack_integration.selecting_bot_types.detailed_explanation')}
-            <i className="fa fa-external-link ml-1" aria-hidden="true"></i>
+          <a className="ml-2 btn-link small" href={t('admin:slack_integration.docs_url.slack_integration')} target="_blank" rel="noopener noreferrer">
+            <i className="icon icon-question ml-1" aria-hidden="true"></i>
           </a>
         </h2>