Discourse forum in documentation as button

[RemDelaporteMathurin]

I saw that this documentation page had a Discourse button on top.

It would be nice to do the same! Maybe Slack too?

https://github.com/jupyterhub/mybinder.org-user-guide/blob/1036fa5f1b527426ed4a902abb7761c4149a9985/doc/conf.py#L69-L73