Add Ballista Getting Started documentation

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
It is hard for new users to get started with Ballista since the project was donated to Arrow. We need to provide some documentation to help these users get started with Ballista.

Describe the solution you'd like
This is mostly about improving some docs:

• Top-level README should briefly explain what Ballista is, and link to the Ballista README
• Ballista README should have a Getting Started section and a link to the User Guide sources

Describe alternatives you've considered
None

Additional context
None