feat: add link to our discourse

may be a bit slow because of the redirection but  best to give a stable url ?

Author: Matthieu Coudron

_data/nav.yml

@@ -11,5 +11,7 @@
   url: https://github.com/neovim/neovim
 - title: Documentation
   url: /doc/
+- title: Forum
+  url: http://discourse.neovim.io
 - title: Sponsors
   url: /sponsors/