[web] Change mailing list refs to discourse refs.

Documentation/mainpage.dox

@@ -7,7 +7,7 @@
  * - Wiki: https://svn.ufz.de/ogs
  * - Developer guide: http://docs.opengeosys.org/docs/devguide
  * - Data Explorer (Gui) manual: http://docs.opengeosys.org/assets/releases/head/docs/DataExplorer-Manual.pdf
- * - Mailing list: https://groups.google.com/forum/#!forum/ogs-users
+ * - Discussion forum: https://discourse.opengeosys.org
  * - Jenkins build server: https://svn.ufz.de/hudson
  *
  *

web/content/_index.pandoc

@@ -131,7 +131,7 @@ features:
           url: /docs/devguide
     - headline: Become part of the Community
       textline: |
-        Get in touch with the OpenGeoSys Community via the [Mailing List](https://groups.google.com/forum/#!forum/ogs-users), [GitHub](https://github.com/ufz/ogs) or by [email](mailto:info@opengeosys.org).
+        Get in touch with the OpenGeoSys Community via our [Discussion forum](https://discourse.opengeosys.org), [GitHub](https://github.com/ufz/ogs) or by [email](mailto:info@opengeosys.org).
       links:
         - text: <i class="fab fa-github"></i> GitHub
           url: https://github.com/ufz/ogs

web/content/docs/_index.pandoc

@@ -43,10 +43,4 @@ url = "https://ufz.github.io/styleguide/cppguide.xml"
 text = "OpenGeoSys C++ Style Guide"
 icon = "fal fa-file-check"
 
-[[sections]]
-name = "Mailing List"
-url = "https://groups.google.com/forum/#!forum/ogs-users"
-text = "The mailing lists for OpenGeoSys users and developers are a good place to ask question related to anything OGS."
-icon = "fal fa-mail-bulk"
-
 +++

web/content/ogs-5/index.pandoc

@@ -5,7 +5,7 @@ aliases = [ "/download5/",           # Redirect for Hydrology II tutorial
             "/resources/downloads" ] # Redirect for Hydrology I tutorial
 +++
 
-As OGS-6 is a reimplementation of OGS-5 with new concepts and methodologies there will probably never be complete feature parity. We provide binaries of OpenGeoSys-5 ready for you to use. Support is limited but you may ask questions or report bugs via the [ogs-users](https://groups.google.com/forum/#!forum/ogs-users) mailing list.
+As OGS-6 is a reimplementation of OGS-5 with new concepts and methodologies there will probably never be complete feature parity. We provide binaries of OpenGeoSys-5 ready for you to use. Support is limited but you may ask questions or report bugs via our [discussion forum](https://discourse.opengeosys.org/tags/ogs5) mailing list.
 
 ## Downloads