cli, docs: Add a docs command for doc generation

This took a lot longer than it looks to get right. It's not perfect, but
it now reliably generates documentation which we can put into gohugo.

lang/funcs/wrapped/wrapped.go

@@ -43,6 +43,8 @@ var _ interfaces.Func = &Func{} // ensure it meets this expectation
 // for the function API, but that can run a very simple, static, pure, function.
 // It can be wrapped by other structs that support polymorphism in various ways.
 type Func struct {
+	//*docsUtil.Metadata // This should NOT happen here, the parents do it.
+
 	// Name is a unique string name for the function.
 	Name string