Write architecture overview

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Ballista has a complex architecture and it is hard to understand by just walking through the code.

Describe the solution you'd like
I would like a document in he repo, with diagrams, that explains how everything fits together. Topics to cover:

• BallistaContext (local and remote)
• How BallistaContext wraps DataFusion context and how state is managed from a remote client
• Python bindings
• How scheduler persists execution graphs and tasks
• How schedule & executor co-ordinate on allocating tasks and reporting results back

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.