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/core/math/fortytwo.go

@@ -65,6 +65,7 @@ func init() {
 		//	}
 		//	return nil, fmt.Errorf("can't use return type of: %s", typ.Out)
 		//},
+		D: FortyTwo, // get the docs from this
 	})
 }