Library Guide: Add SQL level user guide:

### Is your feature request related to a problem or challenge?

Part of https://github.com/apache/arrow-datafusion/issues/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 (https://github.com/apache/arrow-datafusion/issues/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_

Inizia dalla pagina esistente all’indirizzo arrow.apache.org/datafusion/library-user-guide/using-the-sql-api.html e rivedi gli esempi in datafusion-examples. Tratta la generazione di LogicalPlans da SQL, l’estensione di SQL con funzioni definite dall’utente o nodi personalizzati e l’esecuzione di SQL in sola lettura e delle modifiche al catalogo, inclusa la issue 7272 collegata. Il lavoro è completato quando la guida SQL API è stata integrata con esempi funzionanti e facilmente individuabili e con i link pertinenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.