Update UDF Library Docs with UDTFs #8545

Is your feature request related to a problem or challenge?

Excited for the new feature added in #8306! This issue would be updating the library docs with UDTF info. https://arrow.apache.org/datafusion/library-user-guide/adding-udfs.html

Describe the solution you'd like

Updated markdown page with docs including UDTF

Describe alternatives you've considered

n/a

Additional context

n/a