Add example of expr apis including simplification and coercion #3740

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

We [programmatically make Exprs in IOx]
Also I think it would be a good time to start documenting / providing examples of working with the DataFusion Expr APIs as they are quite powerful (e.g the rewrite rules are getting quite sophisticated)

I also think writing some better examples will help improve the API / UX experience as I will

I have found several UX / improvements I will file as a part of writing this

Describe the solution you'd like

• Create an expr_api example to showcase the various Expr apis : Add datafusion example of expression apis #3741
• A PR that moves and updates the README in the examples folder to make them more discoverable
• A PR to run examples in separate CI job, run more of them coverage: Run all datafusion-examples in CI tests #3761
• file a ticket simplification rule for (a + const) + const vs a + (const + const)
• File ticket about needing to run the simplifier twice (found Support simplification that requires multiple applications of constant folding / simplification #1160)

Describe alternatives you've considered
Not document and keep all the sweet sweet tech hidden -- lol.

Additional context
See #3709 and #3708