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 8803504..1983557 100644
--- a/www/index.html
+++ b/www/index.html
@@ -23,7 +23,7 @@
   <div class="submenu">
     <label>Quick Links</label>
     <a href="https://libcxx.llvm.org/">libc++</a>
-    <a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libcxx-dev</a>
+    <a href="https://discourse.llvm.org/c/runtimes/libcxx/10">Discourse Forums</a>
     <a href="https://lists.llvm.org/mailman/listinfo/libcxx-commits">libcxx-commits</a>
     <a href="https://bugs.llvm.org/">Bug Reports</a>
     <a href="https://github.com/llvm/llvm-project/tree/main/libcxxabi/">Browse Sources</a>