Document DML statements

[alamb]

Is your feature request related to a problem or challenge?

DataFusion now supports basic INSERT and COPY statements.

Describe the solution you'd like

We should update https://arrow.apache.org/datafusion/user-guide/sql/index.html to reflect this

Examples are in https://github.com/apache/arrow-datafusion/blob/main/datafusion/sqllogictest/test_files/copy.slt and https://github.com/apache/arrow-datafusion/blob/main/datafusion/sqllogictest/test_files/insert_to_external.slt

Describe alternatives you've considered

No response

Additional context

No response

[alamb]

DML writeup -- IIgood examples to follow from.

[spaydar]

I'll work on this