Library Guide: Add Working with Exprs

[alamb]

Is your feature request related to a problem or challenge?

Part of #7014

If we want to have DataFusion used as the core of many new systems, we need it to be as easy as possible for someone to get their idea working on top of DataFusion.

Thanks to @tshauck we now have a basic Library Users Guide ❤️ and this ticket describes expanding it out

Describe the solution you'd like

Fill in the content of https://arrow.apache.org/datafusion/library-user-guide/working-with-exprs.html

We can draw inspiration from https://github.com/apache/arrow-datafusion/blob/main/datafusion-examples/examples/expr_api.rs

Example Outline

1. Creating Exprs from SQL strings
2. Creating Exprs programatically via the fluent API (e.g. lit(5) + col(5))
3. Rewriting Exprs using TreeNode and transform_down
4. Simplifying expressions (e.g. expr_api.rs)

The examples directory holds a bunch more of examples: https://github.com/apache/arrow-datafusion/tree/main/datafusion-examples

Describe alternatives you've considered

No response

Additional context

No response