add top-level serverextension and installation instructions

[ivanov]

Here's what the output looks like with these modest changes

$ jupyter serverextension enable voila
Enabling: voila
- Writing config: /home/pi/.jupyter
    - Validating...
      voila 0.0.13 OK

and

$ jupyter serverextension list
config dir: /home/pi/.jupyter
    voila  enabled
    - Validating...
      voila 0.0.13 OK

[ivanov]

this is a smaller scope version than #161, but makes the serverextension work with just a top-level enable voila, instead of having to use enable voila.serverextension

[SylvainCorlay]

$ jupyter serverextension enable voila

I think that we should probably recommend the --sys-prefix option.

Otherwise, people may be surprised that this is not conda-environment specific or that enabling the extension requires more privileges than installing the package.

[ivanov]

👍 alright, wrote in --sys-prefix as the only preferred way of installing.