Add link to Ubuntu Discourse section in docs (infra) (#1436)

Add link to Ubuntu Discourse section in docs

In order to get support, Checkbox users can go to the dedicated section
on the Ubuntu Discourse. The "Getting support" page is updated to
reflect this.

docs/community/support.rst

@@ -3,7 +3,8 @@
 Getting support
 ===============
 
-If you have any question related to Checkbox, please send an e-mail to our
-`mailing list`_.
+Checkbox has a `dedicated section`_ in the `Ubuntu Discourse`_. If you have any
+question related to Checkbox, please send a message there!
 
-.. _mailing list: https://lists.ubuntu.com/mailman/listinfo/checkbox-devel
+.. _dedicated section: https://discourse.ubuntu.com/c/certification/checkbox/168
+.. _Ubuntu Discourse: https://discourse.ubuntu.com