Replace question issue placeholder with a button

Ref: https://help.github.com/en/github/building-a-strong-community/configuring-issue-templates-for-your-repository#configuring-the-template-chooser

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,6 @@
+# Ref: https://help.github.com/en/github/building-a-strong-community/configuring-issue-templates-for-your-repository#configuring-the-template-chooser
+blank_issues_enabled: false  # default: true
+contact_links:
+- name: Question
+  url: https://groups.google.com/forum/#!forum/sphinx-users
+  about: For Q&A purpose, please use sphinx-users mailing list.

.github/ISSUE_TEMPLATE/question.md

@@ -1,17 +0,0 @@
----
-name: Question
-about: For Q&A purpose, please use https://groups.google.com/forum/#!forum/sphinx-users
-title: For Q&A purpose, please use sphinx-users group
-labels: 'question'
-assignees: ''
-
----
-
-# Important
-
-This is a list of issues for Sphinx, **not a forum**.
-If you'd like to post a question, please move to sphinx-users group.
-https://groups.google.com/forum/#!forum/sphinx-users
-
-Thanks,
-