Add installation instructions to Python user guide

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
We have no installation instructions for PyBallista

**Describe the solution you'd like**
Add documentation based on installing from PyPi.

We need to do https://github.com/apache/datafusion-ballista/issues/1120 first

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.