stable/2025: fix a run_xrst.sh --external_links error.

appendix/whats_new/2012.xrst

@@ -27,6 +27,8 @@
   uninitialised
   valgrind
   yyyymmdd
+  libeigen
+  gitlab
 }
 
 Release Notes for 2012
@@ -437,9 +439,13 @@ This has since been removed; see
 07-01
 =====
 
+.. _Using custom scalar types: https://
+   libeigen.gitlab.io/docs/TopicCustomizing_CustomScalar.html
+
 #. Change :ref:`cppad_eigen.hpp-name`
    to match new specifications and example in eigen help files on
-   `Using custom scalar types. <https://eigen.tuxfamily.org/dox/TopicCustomizing_CustomScalar.html>`_
+   `Using custom scalar types`_ . 
+
 #. Fix bug whereby a newly constructed :ref:`VecAD-name` object was a
    :ref:`con_dyn_var@Variable` (instead of a parameter) directly after construction
    (when no previous :ref:`ADFun<fun_construct-name>` object had been created).