[libc++][docs] Remove mention of Phabricator on the landing page

Author: ldionne

libcxx/docs/index.rst

@@ -221,9 +221,8 @@ can ask for support on the `libcxx forum`_ or on IRC.
 
 **Patches**
 
-If you want to contribute a patch to libc++, the best place for that is
-`Phabricator <https://llvm.org/docs/Phabricator.html>`_. Please add `libcxx-commits` as a subscriber.
-Also make sure you are subscribed to the `libcxx-commits mailing list`_.
+If you want to contribute a patch to libc++, please start by reviewing our
+:ref:`documentation about contributing <ContributingToLibcxx>`.
 
 **Discussion and Questions**