Add dependencies and configuration changes to allow a build that looks like RTD

Needed these changes in order to preview the RTD site with the changes by @jberlin to this MR: !297 (closed)

Ideally this wouldn't be necessary if we integrate GitLab and RTD, as RTD should be able to create previews from merge requests automatically. But this could be useful anyway (offline environments, quick tests, etc.).

To review this MR, just apply these changes locally and try the make html. That should render the AS docs locally just like in RTD 👍

Cheers Bruno