Update datafusion-examples README

[alamb]

Which issue does this PR close?

Closes #3740

Rationale for this change

Make the examples easier to discover for new users by increasing their documentation

What changes are included in this PR?

1. Update readme (see rendered here https://github.com/alamb/arrow-datafusion/tree/alamb/expr_readme/datafusion-examples)
2. Add newly added example from MINOR: Optimizer example and docs, deprecate Expr::name #3788 into CI

Are there any user-facing changes?

Better docs

[ursabot]

Benchmark runs are scheduled for baseline = 85e8fe1 and contender = c989b8e. c989b8e is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java