docs: add description for Saltelli sampling

docs/source/sampler/saltelli.rst

 Saltelli Sampling
 =================
 
+Saltelli sampling is a method to draw samples for the purpose of Sobol'
+sensitvity analysis (variance-based sensitivity analysis). Detailed description
+of the theory can be found in :cite:t:`Saltelli2002` and :cite:t:`Saltelli2010`.
+The :class:`.Saltelli` class relies on the Python package `SALib` :cite:p:`Herman2017`.
+
 Saltelli Class
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