Change links from Google group to GitHub discussions (CoolProp#2214)

As discussed here:
https://groups.google.com/g/coolprop-users/c/l_aY4vTT58Q

Author: raphaeltimbo

Readme.rst

@@ -16,7 +16,7 @@ It was originally developed by Ian Bell, at the time a post-doc at the Universit
 
 * The documentation is available for the `latest release <http://www.coolprop.org>`_ and the `development version <http://www.coolprop.org/dev>`_  
 
-* For any kind of question regarding CoolProp and its usage, you can ask the `CoolProp user group <https://goo.gl/Pa7FBT>`_ 
+* For any kind of question regarding CoolProp and its usage, you can ask the `CoolProp Discussions <https://github.com/CoolProp/CoolProp/discussions>`_
 
 * ... you might also find answers in our `FAQ <https://github.com/CoolProp/CoolProp/blob/master/FAQ.md>`_ 
 

Web/index.rst

@@ -52,7 +52,7 @@ See more examples of PropsSI usage at :ref:`High-Level interface <high_level_api
 Help
 ----
 
-* (**General Discussion**) Email the `Google group <https://groups.google.com/d/forum/coolprop-users>`_
+* (**General Discussion**) Create a new discussion at `Github CoolProp Discussions <https://github.com/CoolProp/CoolProp/discussions>`_
 * (**Bugs, feature requests**) File a `Github issue <https://github.com/CoolProp/CoolProp/issues>`_
 * `Docs for v4 of CoolProp <http://www.coolprop.org/v4/>`_
 * `Docs for development version of CoolProp <http://www.coolprop.org/dev/>`_