doc/guile-ssh.texi: Improve the "Examples" chapter

* doc/guile-ssh.texi: Add an overview of programming with Guile-SSH to
  the "Examples" chapter.
* doc/examples.texi: Add to the repository.
* doc/Makefile.am (guile_ssh_TEXINFOS): Add `examples.texi'.
* doc/version.texi: Update.
* NEWS: Update.