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ARROW-9757: [Rust] [DataFusion] Add prelude.rs

Users can now just add `use datafusion::prelude::*;` to bring in some key items such as `ExecutionContext` to make it easier to get started.

Rustdocs updated and improved as well.

Closes #7978 from andygrove/prelude

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>