Update README and other meta information to point to the DataCebo forum

[npatki]

Problem Description

For help with troubleshooting or general Copulas questions, we are encouraging users to use our DataCebo forum rather than our Slack. We should update the README and other information to match these expectations.

Expected behavior

1. At the top of the README, change the Community | Slack button to instead say Forum | Join now!. And point it to https://forum.datacebo.com/
2. In the README's "Community & Support" section, replace the text after Questions or issues? with: Join our forum to discuss Copulas features, ask questions, and receive help.
3. In the Contributing guide, replace the URL for "file an issue" with the forum link: https://forum.datacebo.com/
4. In pyproject.toml, the Chat URL should be updated to our forum https://forum.datacebo.com/