New update of doxygen documentation

doc/introduction.dox

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+@page tutorial The Tutorial
+@section introduction Introduction
+
+libssh is a C library that enables you to write a program that uses the
+SSH protocol. With it, you can remotely execute programs, transfer
+files, or use a secure and transparent tunnel for your remote programs.
+The SSH protocol is encrypted, ensures data integrity, and provides strong
+means of authenticating both the server of the client. The library hides
+a lot of technical details from the SSH protocol, but this does not
+mean that you should not try to know about and understand these details.
+
+libssh is a Free Software / Open Source project. The libssh library
+is distributed under LGPL license. The libssh project has nothing to do with
+"libssh2", which is a completly different and independant project.
+
+This tutorial concentrates for its main part on the "client" side of libssh.
+To learn how to accept incoming SSH connexions (how to write a SSH server),
+you'll have to jump to the end of this document.
+
+This tutorial describes libssh version 0.5.0. This version is the development
+version and is *not* published yet. However, the examples should work with
+little changes on versions like 0.4.2 and later.
+
+
+Table of contents:
+
+@subpage guided_tour
+
+@subpage authentication
+
+@subpage shell
+
+@subpage commands
+
+@subpage sftp
+
+@subpage tbd
+
+