Add other smt projects links

Author: relf

doc/index.rst

@@ -15,6 +15,11 @@ SMT is designed to make it easy for developers to implement new surrogate models
 
 The code is available open-source on `GitHub <https://github.com/SMTorg/SMT>`_ together with `introducing tutorials <https://github.com/SMTorg/smt/tree/master/tutorial>`_. 
 
+Other toolboxes linked to SMT are available:
+ - `smt-optim <https://github.com/SMTorg/smt-optim>`_ for multi-fidelity and multi-objective Bayesian optimization with or without constraint
+ - `smt-design-space-ext <https://github.com/SMTorg/smt-design-space-ext>`_ to handle   hierarchical variables in the design space
+ - `smt-explainability <https://github.com/SMTorg/smt-explainability>`_ to provide a collection of model-agnostic explainable AI  methods
+
 Cite us
 -------
 To cite SMT 2.0: P. Saves and R. Lafage and N. Bartoli and Y. Diouane and J. H. Bussemaker and T. Lefebvre and J. T. Hwang and J. Morlier and J. R. R. A. Martins.