Add v2.3.4 to index.rst

Author: rhshadrach

doc/source/whatsnew/index.rst

@@ -16,6 +16,7 @@ Version 2.3
 .. toctree::
    :maxdepth: 2
 
+   v2.3.4
    v2.3.3
    v2.3.2
    v2.3.1