Remove references to old mailing lists that have moved to discourse. Replace with links to discourse.

Reviewed By: #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D129675

GitOrigin-RevId: 44f81dfba407c82589abbb5867714ad030d1b80c
diff --git a/www/index.html b/www/index.html
index 35a6aa8..cf02a0a 100644
--- a/www/index.html
+++ b/www/index.html
@@ -45,9 +45,9 @@
 
 <tt>git clone https://github.com/llvm/llvm-project.git</tt> (<a href="https://github.com/llvm/llvm-project/tree/libclc">View sources</a>)
 
-<h2>Mailing List</h2>
+<h2>Discourse forums</h2>
 
-libclc-dev@lists.llvm.org (<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/libclc-dev">subscribe/unsubscribe</a>, <a href="https://lists.llvm.org/pipermail/libclc-dev/">archives</a>)
+<a href="https://discourse.llvm.org/c/runtimes/opencl/65">Runtimes - OpenCL category</a>
 
 </body>
 </html>