This proposed change tries to clean up a bit of confusion about the server extension workflow.

I was running into an issue like https://github.com/voila-dashboards/voila/issues/561 . However, I wanted to keep my Jupyter install as a system package on Arch (obligatory "arch, btw!") rather than switching to conda. Thus, the addition of `--sys-prefix` to `jupyter serverextension enable` was not an option for me.

There's no Arch/AUR package for Voila just yet (I might make one later), so I'm using Voila via `pip install --user`.

However, I found out in https://github.com/voila-dashboards/voila/blob/88d55eaa/.binder/postBuild that `jupyter serverextension` and `jupyter server extension` are two different concepts. I tried running `jupyter server extension enable --user voila`, and, well... voila! It works now.

So I'd like to propose this change as a "hey, this is confusing in the documentation; perhaps let's change it up in places!" idea.