Re-enable GitHub Discussions in `.asf.yaml` to access past discussions

## Feature Request

**For English only**, other languages will not be accepted.

Please pay attention on issues you submitted, because we maybe need more details.
If no response anymore and we cannot make decision by current information, we will **close it**.

Please answer these questions before submitting your issue. Thanks!

### Is your feature request related to a problem?

- No.

### Describe the feature you would like.

- Re-enable GitHub Discussions in https://github.com/apache/shardingsphere/blob/master/.asf.yaml to access past discussions. See https://github.com/apache/infrastructure-asfyaml/blob/main/README.md#repository-features .
```yaml
github:
features:
discussions: true
notifications:
discussions: notifications@shardingsphere.apache.org
```
- I am not sure if `notifications@shardingsphere.apache.org` can be used; if not, we will need to request a new mailing list from ASF Infra.
- ShardingSphere's GitHub Discussions section contains historical discussions that originated from issues; in the past, some issues were converted into discussions. Being able to revisit these GitHub Discussions would clearly facilitate issue tracking.
- I’m not sure what the final form of https://github.com/apache/shardingsphere/issues/39376 will be, but if AI agent swarms need a dedicated space for discussion, GitHub Discussions is a convenient option for testing.

Start with .asf.yaml and the ASF Infra asfyaml README linked in the issue. Check how repository features and discussion notifications are configured, and verify whether notifications@shardingsphere.apache.org is usable or requires an ASF Infra mailing-list request. Done means GitHub Discussions is re-enabled and its notification destination is valid.

Written by the indexing model from the issue text.