Add example for using a separate threadpool for CPU bound work

[alamb]

TODOs:

• Add some example of trying to do IO on an object store (and should error with no io registered)
• Contemplate adding the DedicatedExecutor code to make the example simple

Which issue does this PR close?

• Closes Document DataFusion Threading / tokio runtimes (how to separate IO and CPU bound work) #12393

Rationale for this change

I added documentation that explains the problem here:

• Improve documentation (and ASCII art) about streaming execution, and thread pools #13423

But now we need to show people how to fix it

I am reminded when trying to do this how non obvious it is

What changes are included in this PR?

Add a well commented example of how to use mutiple runtimes

Are these changes tested?

By CI

Are there any user-facing changes?