Add version 4 link

Author: dpvc

conf.py

@@ -112,6 +112,16 @@
 # A list of ignored prefixes for module index sorting.
 #modindex_common_prefix = []
 
+#
+# DPVC: Add warning about new version
+#
+rst_prolog = """.. attention::
+    `Version 4 <https://docs.mathjax.org/en/latest/index.html>`_ is now the current version of MathJax.
+    This document is for version 3.2.
+"""
+#
+# /DPVC
+#
 
 # -- Options for HTML output ---------------------------------------------------