Docs: clarify what pkgs need to be installed in the Python env on worker nodes

### Proposed change
The current install instructions seem to assume that a single Python environment is shared between the Spawner and the per-user sessions. To help those working on systems where that's not the case it would be useful to clarify:
- what needs installing in the Python env that the spawner runs from
- what needs to be present in the Python env on worker nodes in order to start single-user sessions using BatchSpawner

### Alternative options
?

### Who would use this feature?
All new BatchSpawner admins

### (Optional): Suggest a solution
I'm still in the process of configuring BatchSpawner for the first time so don't wish to propose a solution myself just yet.