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ARROW-11962: [Rust][DataFusion] Improve DataFusion docs

It might be easiest to see the changes by looking at the rendered markdown here: https://github.com/alamb/arrow/tree/alamb/update_datafusion_docs/rust/datafusion

# Rationale
1. It would be nice to market / explain DataFusion a bit more and explain what it is good for

# Changes
1. Describe usecases for DataFusion (Add some marketing "spin"??)
2. Add links to other projects using DataFusion
3. Add links to tech talks about DataFusion
3. Split out developer docs into their own file
4. Use the cool new image that came in https://github.com/apache/arrow/pull/9630

Closes #9701 from alamb/alamb/update_datafusion_docs

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