DOC: update links to forum

CONTRIBUTING.md

@@ -17,7 +17,7 @@ use a self-hosted GitLab instance:
 
 [`https://gitlab.orfeo-toolbox.org/orfeotoolbox/otb`](https://gitlab.orfeo-toolbox.org/orfeotoolbox/otb)
 
-Remember to check out also our [developers mailing list](https://groups.google.com/forum/?hl=fr#!forum/otb-developers/join),
+Remember to check out also our [forum](https://forum.orfeo-toolbox.org/),
 where we discuss some features, improvements and high level project planning.
 You are welcome to ask questions there as a beginner or future OTB contributor!
 

Documentation/Cookbook/rst/index.rst

@@ -32,7 +32,7 @@ Get started now with the :doc:`Installation` chapter.
 
 Get help, share your experience and contribute to the Orfeo-Toolbox project by
 joining `our community <https://www.orfeo-toolbox.org/community/>`_ and
-`users mailing list <https://groups.google.com/forum/?hl=fr#!forum/otb-users/join>`_.
+`users forum <https://forum.orfeo-toolbox.org/>`_.
 
 For other documentation, be sure to read:
 

Documentation/SoftwareGuide/Latex/Introduction.tex

@@ -224,15 +224,15 @@ and its associated data. This data is available at
 \index{OTB!mailing list}
 \index{mailing list}
 
-It is strongly recommended that you join the users mailing list. This is one
+It is strongly recommended that you join the users forum. This is one
 of the primary resources for guidance and help regarding the use of the
-toolkit. You can subscribe to the users list online at
+toolkit:
 
 \begin{center}
-\url{http://groups.google.com/group/otb-users}
+\url{https://forum.orfeo-toolbox.org/}
 \end{center}
 
-The otb-users mailing list is also the best mechanism for expressing your
+The forum is also the best mechanism for expressing your
 opinions about the toolbox and to let developers know about features that you
 find useful, desirable or even unnecessary. OTB developers are committed to
 creating a self-sustaining open-source OTB community. Feedback from users is