[Docs] Add info on type system (DFSchema/Schema) to user guide

[andygrove]

Is your feature request related to a problem or challenge?

A source of confusion for new users is why we have Schema/SchemaRef as well as DFSchema/DFSchemaRef and how to convert between the two.

Here is a quick answer I gave to someone recently. Would be good to add a section about the type system to the user guide, perhaps before introducing TableProvider.

DFSchema is similar to Schema, but adds optional qualifiers (table names). You can convert from Schema by calling DFSchema::try_from_qualified_schema(Some("table_name"), arrow_schema). There is also DFField::from_qualified.

Describe the solution you'd like

More docs

Describe alternatives you've considered

No response

Additional context

No response

[alamb]

Other things we should add:
doc examples of how to go back and forth between Schema. <--> DFSchema

[alamb]

(BTW I aspire to work on this ticket at some point soon)

[caicancai]

take