Update Documentation with `SessionContextExt`

[milenkovicm]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

With introduction of SessionContextExt we would been to update documentation to reflect the change

Describe the solution you'd like

• [ ] update quick start and rust guides at, once we get #1102, #1103 in and from changelogs in #1096 , #1088 https://datafusion.apache.org/ballista/user-guide/deployment/quick-start.html https://datafusion.apache.org/ballista/user-guide/rust.html
• [ ] update configuration options once we get #1104 https://datafusion.apache.org/ballista/user-guide/configs.html
• [ ] update python: https://datafusion.apache.org/ballista/user-guide/python.html once we get #1091 #1100 merged

Describe alternatives you've considered

Additional context

It would be great if we could get README.MD trimmed a bit, with example how to setup and make DataFusion distributed at the very top of the readme.

[tbar4]

@milenkovicm can you assign this to me? I just spent more time than I would like using this documentation as the basis for #1100 instead of the new updated SessionState/Config in SessionState

[milenkovicm]

@tbar4 i have started drafting changes in https://github.com/milenkovicm/arrow-ballista/tree/doc_update_root_readme i did not push it for review yet as #1108 not merged. lets get that merged first and once we start removing ballista context we could update docs ... btw I have no right to assign tasks to anyone ... just put your name in comment and thats it i believe