Add missing cast documentation

Author: serge-sans-paille

docs/source/api/cast_index.rst

@@ -0,0 +1,35 @@
+.. Copyright (c) 2016, Johan Mabille, Sylvain Corlay
+
+   Distributed under the terms of the BSD 3-Clause License.
+
+   The full license is in the file LICENSE, distributed with this software.
+
+.. raw:: html
+
+   <style>
+   .rst-content table.docutils {
+       width: 100%;
+       table-layout: fixed;
+   }
+
+   table.docutils .line-block {
+       margin-left: 0;
+       margin-bottom: 0;
+   }
+
+   table.docutils code.literal {
+       color: initial;
+   }
+
+   code.docutils {
+       background: initial;
+   }
+   </style>
+
+
+Cast
+====
+
+.. doxygengroup:: batch_conversion
+   :project: xsimd
+   :content-only: