[DOC] Generated documentation for AST expression support

[jlowe]

With the addition of AST expression support in #3165, we need the ability to generate documentation describing what is supported by an AST expression. We will also have to consider how best to present this in a way that doesn't confuse users with respect to normal GPU expression support.

[revans2]

We already have a section for each expression saying if it works as a part of lambda or not. Right now nothing works with lambda. But AST is not a one to one mapping to lambda. For some lambda operations, like ArrayTransform, we can do it without any AST support. But we could also do it with AST support. Where as others, like ArraySort, I don't see any good way to do it fully without AST support.

To me it feels like we almost want to replace the Lambda section with an AST section, and then have a way in the docs to explain what operations, expressions or variable require AST support. This could then be expanded in the future to possibly include some hybrid AST/Non-AST support.

So for example with Add we could update it to look like

Expression	SQL Functions(s)	Description	Notes	Context	Param/Output	BOOLEAN	BYTE	SHORT	INT	LONG	FLOAT	DOUBLE	DATE	TIMESTAMP	STRING	DECIMAL	NULL	BINARY	CALENDAR	ARRAY	MAP	STRUCT	UDT
Add	`+`	Addition	None	project	lhs		S	S	S	S	S	S				PS max DECIMAL precision of 18			NS
					rhs		S	S	S	S	S	S				PS max DECIMAL precision of 18			NS
					result		S	S	S	S	S	S				PS max DECIMAL precision of 18			NS
				AST	lhs		NS	NS	S	S	S	S				NS			NS
					rhs		NS	NS	S	S	S	S				NS			NS
					result		NS	NS	S	S	S	S				NS			NS

And for join when we do support parameters for SparkPlan execs we could have something more like

Executor	Description	Notes	Param	BOOLEAN	BYTE	SHORT	INT	LONG	FLOAT	DOUBLE	DATE	TIMESTAMP	STRING	DECIMAL	NULL	BINARY	CALENDAR	ARRAY	MAP	STRUCT	UDT
BroadcastHashJoinExec	Implementation of join using broadcast data	None	conditional (AST param)	S