apacheGH-708: Enable GitHub discussions (apache#710)

## What's Changed

Enable GitHub Discussions like other Arrow repositories.

Fixes apache#708.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: 2 people

.asf.yaml

@@ -29,12 +29,14 @@ github:
     rebase: false
     squash: true
   features:
+    discussions: true
     issues: true
   protected_branches:
     main:
       required_linear_history: true
 notifications:
   commits: commits@arrow.apache.org
+  discussions: user@arrow.apache.org
   issues_status: issues@arrow.apache.org
   issues_comment: github@arrow.apache.org
   pullrequests: github@arrow.apache.org