Library Guide: Add SQL level user guide:

[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/using-the-sql-api.html

Example outline

1. Generating LogicalPlans from sql
2. Example / guide of how to extend SQL (both links to user defined functions and custom nodes, but also maybe some links to the datafusion sql variant parser
3. Example of how to run SQL read only and how to run it with catalog modifications (Misleading/invalid documentation on SessionContext::sql and SessionConfig::create_logical_plan #7272)

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

Describe alternatives you've considered

No response

Additional context

No response