Gitter -> Discord

Author: DimitrisJim

_config.yml

@@ -41,8 +41,8 @@ navigation:
     url: /docs
   - title: GitHub
     url: https://github.com/RustPython/RustPython/
-  - title: Gitter
-    url: https://gitter.im/rustpython/Lobby
+  - title: Discord
+    url: https://discord.com/invite/vru8NypEhv
 
 dashboards:
   - title: CPython test compatibility