[doc] added links to discord and discourse

Some folks are not aware that we have a Discourse server in addition to the mailing lists and a Discord server in addition to IRC. So I think we should add that.
These were announced on the mailing list a while ago: https://lists.llvm.org/pipermail/llvm-dev/2019-November/136880.html

Differential Revision: https://reviews.llvm.org/D100943