User guide should have a reference listing all the built-in functions

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As a user of DataFusion, how do I know what functions are available?

Describe the solution you'd like
User guide should have a reference listing all the built-in functions.

They are documented in crates.io at https://docs.rs/datafusion-expr/9.0.0/datafusion_expr/expr_fn/index.html so maybe we can link to that for now but it would be better if we could generate the same information directly into the user guide.

Describe alternatives you've considered
None

Additional context
None

[alamb]

I think we now have this: https://datafusion.apache.org/user-guide/sql/scalar_functions.html, https://datafusion.apache.org/user-guide/sql/aggregate_functions.html , https://datafusion.apache.org/user-guide/sql/window_functions.html