[Doc] replace github discussion link with discourse (#12684)

README.rst

@@ -298,13 +298,13 @@ Getting Involved
 ----------------
 
 - `Community Slack`_: Join our Slack workspace.
-- `GitHub Discussions`_: For discussions about development, questions about usage, and feature requests.
+- `Forum`_: For discussions about development, questions about usage, and feature requests.
 - `GitHub Issues`_: For reporting bugs.
 - `Twitter`_: Follow updates on Twitter.
 - `Meetup Group`_: Join our meetup group.
 - `StackOverflow`_: For questions about how to use Ray.
 
-.. _`GitHub Discussions`: https://github.com/ray-project/ray/discussions
+.. _`Forum`: https://discuss.ray.io/
 .. _`GitHub Issues`: https://github.com/ray-project/ray/issues
 .. _`StackOverflow`: https://stackoverflow.com/questions/tagged/ray
 .. _`Meetup Group`: https://www.meetup.com/Bay-Area-Ray-Meetup/