Library Guide: Add Using the DataFrame API

[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-dataframe-api.html

Example Outline

1. Creating DataFrame and how it is a thin wrapper over LogicalPlan
2. Discuss collect and streaming output
3. Writing DataFrame to files
4. How to go back and forth between LogicalPlan and DataFrame

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

[Veeupup]

@alamb It seems like a very basic but important topic. I can help with the doc : )

[alamb]

Thank you @Veeupup