Slack and discussion links broken

## ❓ Questions and Help Hi maintainers, I was trying to reach out to the maintainers and found that the links to slack on Github and the website are broken. When that didn't work, I tried the discussion page (https://discuss.dgl.ai/) and that didn't work either. Are those no longer used? I'm happy to post my questions to github, but I wanted to try those other methods first.