tutorial: move introductory text into introduction (could be missed otherwise)

doc/source/tutorial/index.rst

 Tutorial: Learning Kwant through examples
 =========================================
 
-In the following, the most important features of Kwant are explained using
-simple, but still physically meaningful examples. Each of the examples
-is commented extensively. In addition, you will find notes about more subtle,
-technical details at the end of each example. At first reading,
-these notes may be safely skipped.
-
 .. toctree::
     introduction
     tutorial1

doc/source/tutorial/introduction.rst

 Introduction
 ============
 
+In this tutorial, the most important features of Kwant are explained using
+simple, but still physically meaningful examples. Each of the examples is
+commented extensively. In addition, you will find notes about more subtle,
+technical details at the end of each example. At first reading, these notes may
+be safely skipped.
+
 Quantum transport
 .................