Add KDocs for groupBy/pivot

[Jolanrensen]

Related to #811

The groupBy operation is one of the most advanced operations in DataFrame. It has an extensive DSL that is only documented on the website at the moment. It's a hard task but one we must do eventually.

The same holds for the pivot operation. Let's try to make people not scared of this advanced function anymore :)

I've grouped them together in one issue because they call into each other's DSL. This needs to be shown in the KDocs and the grammar DSLs.

Take note of aggregate and all the functions inside it too. This could link to a new common "statistics" definition.
A common default() definition could also likely be created :)