Add documentation on extending Ballista

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
It would be good to have some documentation explaining how to extend Ballista to add custom object stores and other functionality.

Describe the solution you'd like
Add documentation in the user guide explaining how to extend Ballista. The simplest approach is to use ballista as a library and provide your own executor and scheduler processes that set up initial state and then invoke Ballista.

Describe alternatives you've considered
None

Additional context
None