Automate production of SQL and DataFrame references for SQL functions / Expressions

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
We are currently manually maintaining the user guide. In the same way that we generate the configuration section, we could generate much of the content in the SQL and DataFrame references.

Describe the solution you'd like
Define documentation in code and then generate markdown. The definition in the code can have SQL and DataFrame-specfic usage examples.

Describe alternatives you've considered
None

Additional context
None