Consolidate dataframe example

[alamb]

Which issue does this PR close?

• Related to Epic: Better / Improved Documentation, Tutorials and Examples #7013

Rationale for this change

I am in the process of adding more examples, so I am trying to improve the landing area

Each example builds a binary / requires creating a different binary (100MB in debug builds) as well as making it harder to find relevant examples. Keeping the number of examples smaller makes it easier to find what you are looking for.

What changes are included in this PR?

1. Consolidate dataframe.rs and dataframe_in_memory.rs into a single file
2. Improve documentation / add commentary

Are these changes tested?

By CI/Clippy

Are there any user-facing changes?

one fewer example, easier to read

[comphead]

lgtm thanks @alamb

[alamb]

Thanks @comphead and @crepererum

In case anyone cares, here is my new example I am adding:

• Add example for using a separate threadpool for CPU bound work #13424