Add DataFrame section to user guide

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The user guide has a SQL section but no DataFrame section.

Describe the solution you'd like
Add a new DataFrame section.

• list all dataframe functions
• list all logical expression functions

We should explain how to use DataFrame and logical expressions. We can also link to https://docs.rs/datafusion/latest/datafusion/dataframe/struct.DataFrame.html from here

Describe alternatives you've considered
None

Additional context
None