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ARROW-11969: [Rust][DataFusion] Improve Examples in documentation

# Rationale

When people find `datafusion` on crates.io they want both a textual description of what it does as well as some example code to see if it would help them. For example, look at how tokio does it: https://crates.io/crates/tokio

# Changes
1.  Add an example on the main README.md of datafusion (that appears on the crates.io homepage) that shows a prospective user what DataFusion offers, lifted from the existing example from https://docs.rs/datafusion/3.0.0/datafusion
2. Add formatted output to the `rustdoc` examples in the main `lib.rs` page to show better what the output is
3. Fix some rustdoc warnings I noticed while testing

Closes #9710 from alamb/alamb/better_examples

Authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>