Write architecture overview guide:

[alamb]

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

I would like to continue to grow / scale the community of users and contributors to DataFusion and part doing so is to help them both:

1. Get up to speed contributing quickly
2. Quickly determine if DataFusion has the appropriate architecture for their needs

I do not think the current architecture docs https://github.com/apache/arrow-datafusion#architecture-overview are sufficient for this case

Describe the solution you'd like
I would like a high level writeup of the architecture . It will be a balancing act to describe the architecture in enough detail to be helpful but not so detailed that it get out of date quickly.

Describe alternatives you've considered

Additional context
@xudong963 mentioned this in #980 as well

[alamb]

THe kind of outline I was thinking was:

Expr

LogicalPlan

PhysicalPlan

PhysicalExpr

Execution Model (aka document the ExecutionPlan API)

Testing

[alamb]

This is covered by #5501 I think , closing this as a dupe