Add Greek translation of the tutorial

Add Greek translation of the tutorial

* etc/tutorials/TUTORIAL.el_GR: Add tutorial in Greek.  It is a
faithful translation of the TUTORIAL.
* etc/tutorials/TUTORIAL.translators (Author): Mention myself as
the author and the maintainer.
* etc/NEWS: Announce it.  (Bug#55314)

* lisp/language/greek.el (set-language-info-alist): Link to the
tutorial and include sample text.